Reference

The Rare Disease & Clinical Trial Glossary

1,920 terms your doctor, trial coordinator, and the FDA use, defined the way a person would explain them. These are the same definitions that appear as hover explanations across Trial Friend. Every term has its own link, so you can send someone straight to the one they need.

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2,3-DPG: 2,3-diphosphoglycerate, a molecule in red blood cells that loosens hemoglobin's grip on oxygen. High levels are linked to more sickling.
5q SMA: The common form of spinal muscular atrophy, caused by a fault in the SMN1 gene on chromosome 5. It accounts for roughly 95% of SMA cases.
6MWT: 6-minute walk test — measures how far you can walk in 6 minutes to assess your functional endurance and exercise capacity.

A

A2E: A toxic byproduct of the visual cycle that accumulates in the retinal pigment epithelium in Stargardt disease, contributing to photoreceptor death.
AATD: Alpha-1 antitrypsin deficiency: an inherited shortage of working alpha-1 antitrypsin, a liver protein that protects the lungs. It can cause early emphysema and liver damage.
AAV: Adeno-Associated Virus, a small virus commonly used as a delivery vehicle in gene therapy because it rarely causes disease.
AAV8: A harmless engineered virus shell (adeno-associated virus type 8) used to deliver gene therapy into cells, with a strong preference for the liver.
AAV9: A harmless engineered virus shell (adeno-associated virus type 9) used to deliver gene therapy, notable for reaching the brain and spinal cord.
ABCA4: A gene that encodes a protein responsible for clearing toxic byproducts (like A2E) from photoreceptor cells. Mutations in ABCA4 cause Stargardt disease and some forms of cone-rod dystrophy.
abciximab: An older antibody drug (ReoPro) that blocks platelets from clumping, once widely used during heart procedures.
ablation: A procedure that destroys abnormal tissue using heat, cold, or electrical energy. Used to treat tumors or correct heart rhythm problems.
accelerated approval: An FDA process allowing approval based on preliminary evidence, with further testing continuing after approval.
aceruloplasminemia: A rare inherited disease in which the iron-carrying protein ceruloplasmin is missing, so iron builds up in the brain, pancreas, and other organs over decades.
acetylcholine: A neurotransmitter involved in muscle contraction and many brain functions.
achondroplasia: The most common form of short-limbed dwarfism, caused by a mutation in the FGFR3 gene that restricts bone growth at the growth plates.
AChR: Acetylcholine receptor, the docking site on a muscle that receives a nerve's signal to contract. Antibodies against it cause most cases of myasthenia gravis, and the AChR antibody blood test is usually the first test doctors order.
acid alpha-glucosidase: The enzyme that breaks down glycogen inside lysosomes. Deficiency of this enzyme causes Pompe disease, leading to glycogen buildup in muscles.
acidosis: A condition where the blood becomes too acidic, disrupting normal body functions. Can result from lung or kidney problems, or metabolic imbalances.
acoramidis: An oral TTR stabilizer (brand name Attruby) designed to mimic the protective T119M gene variant. Approved in November 2024 for ATTR cardiomyopathy.
acquired resistance: When cancer cells develop new mutations that allow them to stop responding to a targeted therapy that was previously working.
acrocyanosis: A bluish discoloration of the hands and feet caused by reduced blood flow to the extremities.
acromegaly: A rare hormonal disorder where the pituitary gland produces too much growth hormone in adults, causing enlargement of hands, feet, and facial features.
acroparesthesia: Burning or tingling pain in the hands and feet, a hallmark early symptom of Fabry disease.
ACT-EARLY: BridgeBio's Phase 3 trial (NCT06563895) testing whether acoramidis, sold as Attruby, can delay or prevent ATTR amyloidosis in carriers aged 18 to 75 with no signs of disease. About 600 participants; recruiting.
ACTH: Adrenocorticotropic hormone, made by the pituitary gland at the base of the brain. It tells the adrenal glands to make cortisol.
action date: The deadline by which the FDA has committed to decide on a drug application, also called the PDUFA date.
active phase: The first 6 months to 2 years of thyroid eye disease, when the tissue behind the eye is inflamed and still changing. Drugs are tested in this window; afterwards the disease burns out and surgery addresses what remains.
activin signaling inhibitor: A drug that blocks activin proteins, which can cause blood vessels to thicken and narrow. Used in pulmonary arterial hypertension treatment.
acute: Sudden onset, usually short-term and sometimes severe.
acute chest syndrome: A serious lung complication of sickle cell disease, with chest pain, fever or trouble breathing and a new shadow on a chest X-ray.
ACVR1: The gene for the ALK2 bone-growth receptor. A single-letter ACVR1 mutation makes the receptor overactive, driving the runaway bone formation of FOP.
Adakveo: Brand name of crizanlizumab, an infusion approved in 2019 for sickle cell pain crises. Its label now says a later trial did not establish its benefit.
adalimumab: An anti-inflammatory antibody (brand Humira) that blocks TNF, used across many autoimmune diseases.
adaptive design: A trial design that allows modifications based on preliminary data without compromising validity.
ADC: Antibody-Drug Conjugate. A targeted cancer therapy that attaches a toxic drug to an antibody, delivering the drug directly to tumor cells while reducing damage to healthy tissue.
ADEM: Acute disseminated encephalomyelitis, a sudden episode of inflammation across the brain and spinal cord that causes confusion, drowsiness, weakness and sometimes seizures, most often in children. Many cases are linked to the MOG antibody.
adenocarcinoma: A cancer that starts in gland cells, the cells that line organs and make mucus or other fluids. Most lung, colon, pancreas and prostate cancers are adenocarcinomas.
adenomyosis: A condition where the uterus lining grows into the muscular wall of the uterus, causing heavy, painful periods.
ADH1: Autosomal dominant hypocalcemia type 1, a rare genetic disorder of calcium balance caused by an overactive calcium-sensing receptor.
adhesion: The process by which cells or tissues stick together. Cell adhesion molecules act like molecular glue holding tissue structure intact.
adjuvant: Additional treatment given after the main treatment to lower the chance of recurrence.
adjuvant therapy: Additional treatment given after the primary treatment (usually surgery) to lower the risk of the disease coming back.
adrenal crisis: A life-threatening emergency in adrenal insufficiency where the body cannot produce enough cortisol, causing dangerously low blood pressure and potential cardiovascular collapse.
adrenal insufficiency: A condition where the adrenal glands don't produce enough hormones, especially cortisol. Requires daily steroid replacement and emergency protocols for physical stress.
adverse event: An unwanted side effect or medical problem during treatment.
advisory committee: A panel of outside experts, patients and consumer representatives that the FDA convenes to vote on a drug application. The FDA is not bound by the vote; in 2016 it approved Exondys 51 after the panel voted 7 to 6 against.
aggressive systemic mastocytosis: An advanced form of mastocytosis with rapid progression, organ damage, and bone marrow dysfunction requiring targeted therapy.
aGVHD: Acute Graft-Versus-Host Disease — an early complication after stem cell transplant, typically appearing within the first 100 days.
AHDS: Allan-Herndon-Dudley syndrome, another name for MCT8 deficiency.
alcohol septal ablation: A catheter procedure that injects alcohol into a small artery to deliberately scar and shrink part of the thickened septum, relieving obstruction without open surgery.
aldose reductase: An enzyme that converts glucose into sorbitol. In CMT-SORD, blocking this enzyme with govorestat (AT-007) is being tested as a way to reduce toxic sorbitol buildup.
aldosterone: A hormone produced by the adrenal glands that regulates salt and water balance, directly affecting blood pressure.
Aldurazyme: Brand name for laronidase, the FDA-approved enzyme replacement therapy for mucopolysaccharidosis type I (MPS I).
ALE1: An investigational oral small molecule for hypophosphatasia, designed as a pill alternative to Strensiq enzyme injections. Originally developed by Alesta Therapeutics, now a BioMarin program.
ALK2: A bone-growth signaling receptor (also called ACVR1). Overactive ALK2 signaling drives the runaway bone formation of FOP, and ALK2 inhibitors aim to quiet it.
allele: A variant form of a gene at a specific location on a chromosome.
allogeneic: Using cells or tissue from a donor (not your own).
Alnylam Act: A no-charge genetic testing and counseling program sponsored by Alnylam for people in the US and Canada with a family history or signs of hereditary ATTR amyloidosis. A clinician confirms eligibility and orders the test.
alopecia: Hair loss, which can be partial or complete and affect the scalp or the entire body.
alpha-gal: Short for galactose-alpha-1,3-galactose: a sugar molecule in the tissue of most mammals, but not in humans. In alpha-gal syndrome the immune system treats it as a threat, so foods and ingredients that carry it can trigger reactions.
Alpha-Gal Syndrome: An IgE-mediated allergy to galactose-α-1,3-galactose (alpha-gal), a sugar molecule in non-primate mammalian tissue. Triggered by tick bites (primarily lone star tick) and causes delayed reactions to red meat, dairy, gelatin, heparin, and other mammalian-derived medical products.
alpha-galactosidase A: The enzyme that breaks down a fatty substance called Gb3 (globotriaosylceramide) inside cells. Deficiency of this enzyme causes Fabry disease.
alpha-L-iduronidase: The enzyme missing or deficient in MPS I. Without it, GAG molecules cannot be properly broken down and accumulate in tissues.
alpha-synuclein: A protein that accumulates abnormally in several neurodegenerative diseases.
ALT: Alanine aminotransferase — a liver enzyme. Elevated = possible liver issue.
alternative complement pathway: One of three pathways that activate the complement immune system. Dysregulation of this pathway drives C3 glomerulopathy, PNH, and aHUS.
alveolar macrophage: Immune cells that live inside the lungs' air sacs and act as cleanup crew, removing used surfactant and killing inhaled germs.
alveolitis: Inflammation of the alveoli (tiny air sacs in the lungs), which can impair oxygen exchange and cause breathing difficulty.
amaurosis: Partial or complete loss of vision, particularly temporary blindness, often caused by reduced blood flow to the eye.
Amblyomma americanum: The scientific name for the lone star tick. Native to the southeastern and south-central United States and the primary trigger for Alpha-Gal Syndrome.
ambulation: The ability to walk independently.
ambulatory: Able to walk independently. In clinical trials, ambulatory status is often an eligibility requirement and measured by timed walking tests.
amenable mutation: A genetic mutation that responds to a specific treatment. In Fabry disease, only certain GLA gene mutations are amenable to migalastat (Galafold).
aminoaciduria: Higher-than-normal amounts of amino acids in the urine, a sign that the kidney's filtering tubules are not reabsorbing them properly. Common in Wilson disease.
aminoglycoside: A class of antibiotics (gentamicin, tobramycin, neomycin) that can worsen neuromuscular conditions like myasthenia gravis by impairing nerve-to-muscle signaling.
Amvuttra: Brand name of vutrisiran, Alnylam's RNA interference TTR silencer injected under the skin every 3 months. The only ATTR drug approved for both the nerve and heart forms.
amyloid: Protein that has misfolded into stiff fibers that pile up in tissue. Different proteins form it in different diseases: transthyretin in ATTR amyloidosis, antibody light chains in AL amyloidosis and amyloid-beta in the brain in Alzheimer disease.
amyloidosis: A group of diseases in which misfolded proteins clump into deposits (amyloid) that build up in organs like the heart, kidneys, and nerves.
anakinra: An interleukin-1 receptor antagonist (brand name Kineret) FDA-approved for rheumatoid arthritis and cryopyrin-associated periodic syndromes. Used off-label in sJIA and emergently for macrophage activation syndrome.
anaphylaxis: A severe, potentially life-threatening allergic reaction.
Anaplasma: A genus of bacteria transmitted by ticks that infect white blood cells (neutrophils), causing anaplasmosis.
anaplasmosis: A bacterial infection spread by tick bites, causing fever, chills, and low blood counts. Treatable with antibiotics.
anatomical: Relating to the structure of the body and its organs.
ANC: Absolute Neutrophil Count — measures your infection-fighting white blood cells.
ANCA: Anti-Neutrophil Cytoplasmic Antibody — an autoantibody found in certain vasculitis conditions. About 30–40% of EGPA patients are ANCA-positive (usually anti-MPO).
androgen: Male sex hormones (like testosterone) that control the development of male characteristics and reproductive function.
anemia: A condition where the blood lacks enough healthy red blood cells to carry adequate oxygen to the body's tissues.
aneuploidy: An abnormal number of chromosomes in a cell, which can cause developmental problems or cancer.
aneurysm: Weakening and bulging of an artery wall, which could rupture.
angiogenesis: The formation of new blood vessels, which tumors stimulate to supply themselves with nutrients and oxygen for growth.
angiokeratoma: Small, dark red or purple raised spots on the skin caused by widened, damaged blood vessels. A characteristic sign of Fabry disease.
ANGPTL3: A protein that raises blood fat levels. Blocking ANGPTL3 lowers cholesterol and triglycerides in rare inherited lipid disorders.
Annual Notice of Change: A letter every Medicare drug or Medicare Advantage plan must send members by September 30th listing what changes in January: premiums, deductibles, covered drugs and their rules.
annualized relapse rate: The average number of MS relapses per person per year in a trial, so 0.10 means about 1 relapse every 10 years. It is the usual primary endpoint in relapsing MS trials.
ANS03: An experimental drug from Avistone Biotechnology designed to block both ROS1 and TRK. It is in a first-in-human Phase 1 trial in advanced solid tumors (NCT06716138).
anti-angiogenic: A treatment that blocks the formation of new blood vessels to a tumor, starving it of nutrients.
anti-CD20: Describes an antibody drug that binds CD20, a protein on the surface of B cells, and clears those cells from the blood. Kesimpta, Ocrevus, Briumvi and rituximab are anti-CD20 drugs.
anti-drug antibody: Immune proteins the body produces against a therapeutic drug, which can reduce the drug's effectiveness over time.
anti-MPO: Anti-myeloperoxidase antibody — the type of ANCA most commonly found in EGPA patients who are ANCA-positive.
anti-SSA: An autoantibody (also called anti-Ro) found in about 70% of Sjögren's syndrome patients. Used as a diagnostic marker.
anti-SSB: An autoantibody (also called anti-La) found in about 40% of Sjögren's syndrome patients. Used alongside anti-SSA for diagnosis.
antibody: Proteins produced by the immune system that bind to specific invaders (antigens) to neutralize them or mark them for destruction.
antibody titer: A blood test measuring the concentration of a specific antibody. Higher titers indicate a stronger immune response.
antibody-drug conjugate: A targeted cancer therapy that links a monoclonal antibody to a toxic drug payload. The antibody delivers the drug directly to cancer cells, sparing healthy tissue.
antiemetic: A medication that prevents or treats nausea and vomiting, commonly given alongside chemotherapy.
antifibrotic: A drug that slows the formation of scar tissue. In IPF, the 3 approved antifibrotics (pirfenidone, nintedanib and nerandomilast) slow the loss of lung function but do not reverse existing scarring.
antigen: A substance (usually a protein) that triggers an immune response. Foreign antigens from pathogens are targeted by antibodies.
antisense: A molecular therapy that uses short synthetic nucleic acid sequences (DNA or RNA) to block harmful gene expression.
antisense oligonucleotide: A short piece of synthetic DNA that blocks the production of harmful proteins.
antithrombin: A protein in the blood that shuts clotting down once it is under way. Qfitlia lowers it, and the dose is adjusted so antithrombin activity stays between 15% and 35% of normal.
aorta: The largest artery in the body, carrying oxygen-rich blood from the heart to the rest of the body.
aortic valve: The heart valve between the left ventricle and the aorta that prevents blood from flowing back into the heart.
aPCC: Activated prothrombin complex concentrate, a bypassing agent sold as FEIBA. At high doses alongside Hemlibra it has caused blood clots and thrombotic microangiopathy, the subject of Hemlibra's boxed warning.
aphasia: Loss or impairment of the ability to understand or produce speech, usually caused by brain damage.
apitegromab: An antibody (brand Isembyld) that blocks myostatin, the body's brake on muscle growth. Approved in September 2026 for SMA in people 2 and older who are already on an SMN2-targeted treatment, making it the first muscle-targeted SMA drug.
aplasia: The failure of an organ or tissue to develop normally, or the complete absence of tissue growth.
apnea: A temporary pause in breathing. Sleep apnea involves repeated episodes during sleep.
APOL1: A gene whose high-risk variants, most common in people of West African ancestry, sharply raise the risk of certain kidney diseases.
apoptosis: Programmed cell death, a normal biological process where the body eliminates old, damaged, or unnecessary cells in an orderly way.
APPLAUSE.IgAN trial: A Phase III clinical trial studying iptacopan (Fabhalta) in IgA nephropathy as a complement inhibitor.
apraxia: Difficulty with coordinating purposeful movements despite having normal strength and sensation.
APRIL: A Proliferation-Inducing Ligand, a protein that stimulates immune cells to produce IgA antibodies. Overactivity of APRIL contributes to IgA nephropathy.
APRIL inhibitor: A drug that blocks APRIL, a protein that drives production of the abnormal IgA1 antibodies responsible for IgA nephropathy.
Aqneursa: The brand name for levacetylleucine, IntraBio's oral suspension approved for Niemann-Pick disease type C (2024) and for ataxia in ataxia-telangiectasia (September 18, 2026).
AQP4: Aquaporin-4, a water channel protein on brain support cells. Antibodies against it cause most cases of NMOSD.
Aqvesme: Brand name of mitapivat for anemia in adults with alpha- or beta-thalassemia, approved in December 2025 with a boxed warning for liver injury and a required safety program.
arachnodactyly: Unusually long, thin fingers and toes, seen in connective tissue disorders like Marfan syndrome.
arachnoid: The delicate, web-like middle layer of the three membranes (meninges) that cover the brain and spinal cord.
Aralast NP: Takeda's brand of alpha-1 proteinase inhibitor for AATD augmentation, approved in 2002 and supplied as a powder mixed before infusion.
argyria: A permanent bluish-gray skin discoloration caused by chronic ingestion of silver compounds.
arrhythmia: An abnormal heart rhythm that is either too fast, too slow, or irregular.
arrhythmogenic: Capable of causing abnormal heart rhythms. Arrhythmogenic conditions make the heart prone to dangerous rhythm disturbances.
arteriopathy: Any disease affecting the arteries, such as hardening, inflammation, or abnormal wall structure.
arteritis: Inflammation of the walls of arteries, which can narrow or block blood flow and damage organs.
arthritis: Inflammation of one or more joints, causing pain, swelling, and stiffness that can worsen with age.
arthropathy: Any disease or abnormality affecting a joint.
ASGPR: Asialoglycoprotein receptor, a protein on the surface of liver cells. Some degrader drugs attach their target to ASGPR so the liver pulls the target inside and destroys it.
ASS1: A gene for a urea cycle enzyme (argininosuccinate synthetase 1). ASS1 mutations cause citrullinemia type 1, a disorder of ammonia buildup.
assay: A laboratory test that measures the presence, amount, or activity of a specific substance in a sample.
AST: Aspartate aminotransferase — a liver enzyme. Elevated = possible liver issue.
astigmatism: A common vision condition caused by an irregularly shaped cornea or lens, making both near and far objects appear blurry.
asymptomatic: Having no noticeable symptoms.
AT-007: The development code for govorestat, an aldose reductase inhibitor in clinical trials for CMT-SORD.
atacicept: A drug that blocks BAFF and APRIL, 2 signals that keep the B cells making faulty IgA1 alive. Sold as Trutakna and self-injected once a week for IgA nephropathy.
ataxia: Loss of muscle coordination and control, typically affecting movement and balance.
ataxia-telangiectasia: A rare inherited disorder, abbreviated A-T, in which the ATM gene that repairs DNA does not work. Children lose coordination from early childhood, have weakened immunity and a high risk of blood cancers. Aqneursa became its first approved treatment on September 18, 2026.
Atebrioz: The brand name of zilurgisertib, Mirum's once-daily oral ALK2 inhibitor for fibrodysplasia ossificans progressiva, approved by the FDA on September 25, 2026 for patients 12 and older.
atherosclerosis: Buildup of fatty deposits (plaque) inside artery walls, narrowing them and restricting blood flow.
ATM: The gene (ataxia-telangiectasia mutated) that makes a protein sensing broken DNA and directing its repair. Losing both copies causes ataxia-telangiectasia; carrying one copy raises breast cancer risk.
ATP7A: A gene encoding a copper-transporting ATPase. Mutations in ATP7A cause Menkes disease by preventing proper copper absorption and distribution.
ATP7B: A gene containing the instructions for a copper-exporting protein in liver cells. Mutations in both copies of ATP7B cause Wilson disease, leading to toxic copper buildup.
atresia: The abnormal closure or absence of a body opening or passage that is normally present, such as a heart valve or intestinal segment.
atrium: One of the two upper chambers of the heart that receive blood returning from the body (right) or lungs (left).
atrophy: Shrinkage or wasting of a tissue or organ, usually from disuse, poor nutrition, or disease.
ATTR-CM: Transthyretin amyloid cardiomyopathy: the heart form of ATTR amyloidosis, in which misfolded transthyretin stiffens the heart muscle. It can be wild-type or hereditary.
ATTR-PN: Transthyretin amyloid polyneuropathy: the nerve form of ATTR amyloidosis, causing numbness, pain and autonomic problems. Drugs are approved for the hereditary form only.
ATTRACT trial: An active-controlled comparability study comparing migalastat (Galafold) to enzyme replacement therapy (agalsidase beta) in Fabry disease.
Attruby: Brand name of acoramidis, BridgeBio's oral TTR stabilizer approved in November 2024 for ATTR cardiomyopathy, taken as 2 tablets twice a day.
augmentation therapy: Weekly IV infusions of alpha-1 antitrypsin collected from donated human plasma, used to raise blood AAT levels in people with severe AATD and emphysema.
Augtyro: Brand name for repotrectinib, a next-generation TRK inhibitor approved for NTRK fusion cancers including treatment-resistant cases.
Authorized Treatment Center: An FDA-credentialed hospital or treatment site authorized to administer specific gene therapies (like Casgevy and Lyfgenia) or other complex therapies. Each ATC is trained and qualified by the manufacturer.
autoantibody: Antibodies that mistakenly attack the body's own healthy cells and tissues.
autoimmune: A condition where the body's immune system mistakenly attacks its own healthy cells and tissues.
autoimmune disorder: A disease in which the immune system mistakenly attacks the body's own cells and tissues.
autoimmune hepatitis: A long-term disease in which the immune system attacks the liver, usually treated with steroids and other immune-suppressing drugs.
autoimmune pancreatitis: Inflammation of the pancreas driven by the immune system. Type 1 autoimmune pancreatitis is the most common manifestation of IgG4-related disease.
autologous: Using your own cells or tissue (not from a donor).
autonomic: Relating to the autonomic nervous system, which controls involuntary functions like heart rate, digestion, and blood pressure.
autonomic nervous system: The part of the nervous system that automatically controls functions you do not think about, like heart rate, digestion, and blood pressure.
autophagy: The cell's natural recycling process, where it breaks down and reuses its own damaged or unnecessary components.
autosomal: Inherited through non-sex chromosomes — affects males and females equally.
autosomal dominant: An inheritance pattern in which 1 copy of a disease-causing variant, from either parent, is enough to cause the disease or raise the risk of it. Each child of a carrier has a 50% chance of inheriting it.
autosomal recessive: An inheritance pattern where two copies of a disease gene are needed to cause the disease.
AV block: Atrioventricular block: a delay or interruption in the electrical signal between the heart's upper and lower chambers that can slow the heartbeat and cause fainting.
avapritinib: A selective KIT inhibitor (brand name Ayvakit) approved for advanced systemic mastocytosis and certain GI stromal tumors.
axatilimab: An antibody (brand Niktimvo) that blocks CSF1R on immune cells, approved for chronic graft-versus-host disease.
axon: The long, thin part of a nerve cell that transmits electrical signals.
axonal: Relating to the axon, the long cable-like part of a nerve cell that carries electrical signals from the cell body to its target. Axonal damage in CMT2 is a different problem from the demyelinating damage in CMT1.
axonopathy: Disease or damage to the axons (the long fibers of nerve cells that transmit signals), causing numbness and weakness.
Ayvakit: Brand name for avapritinib, an FDA-approved targeted therapy for advanced systemic mastocytosis driven by KIT D816V mutations.
azathioprine: An older immune-suppressing pill (brand Imuran) used off label to prevent relapses in MOGAD and several other autoimmune conditions.
azotemia: Elevated levels of nitrogen-containing waste products (like urea) in the blood, indicating impaired kidney function.

B

B-cell: A type of white blood cell that produces antibodies to fight infection. Overactive B cells can drive autoimmune diseases.
Babesia: A genus of parasites transmitted by ticks that invade and destroy red blood cells, causing babesiosis.
babesiosis: A malaria-like infection of red blood cells caused by a parasite spread through tick bites.
bacteremia: The presence of bacteria in the bloodstream.
BAFF: B-cell Activating Factor, a protein that helps immune cells survive and produce antibodies. Elevated BAFF is linked to autoimmune kidney diseases.
BAG3: A gene that helps heart muscle cells manage stress. BAG3 mutations cause an inherited form of dilated cardiomyopathy.
BALANCE trial: A Phase III trial comparing Elfabrio (pegunigalsidase alfa) head-to-head against Fabrazyme (agalsidase beta) in Fabry disease.
Bartonella: A genus of bacteria that infect the lining of blood vessels, causing bartonellosis. Transmitted by ticks, fleas, and other vectors.
bartonellosis: Infection with Bartonella bacteria, spread by cat scratches, fleas, or lice. Causes include cat scratch disease and trench fever.
barzolvolimab: An investigational antibody that depletes mast cells by blocking their KIT receptor, studied in chronic hives and mast cell diseases.
basal ganglia: Brain structures involved in controlling movement and coordination.
baseline: The measurement taken before treatment starts, used as the comparison point for judging whether a drug changed anything.
basement membrane: A thin sheet of proteins that anchors surface cells to underlying tissue. Attacked by autoantibodies in some blistering and kidney diseases.
basket trial: A trial that enrolls patients with different diseases sharing a common genetic mutation or biomarker.
batoclimab: An investigational FcRn-blocking antibody that lowers harmful IgG antibodies, studied in myasthenia gravis and thyroid eye disease.
Batten disease: A group of rare, fatal inherited nervous system disorders (neuronal ceroid lipofuscinoses) in which toxic substances build up in brain cells, causing progressive neurological decline.
Bayley-III: Bayley Scales of Infant and Toddler Development, Third Edition. A standard test of how a young child is developing, used in the Fayuvi trial to measure cognitive skills.
BBP-418: BridgeBio's oral ribitol for limb-girdle muscular dystrophy type 2I/R9, under FDA priority review with a decision due November 27, 2026. It would be the first approved drug for any form of LGMD.
BCL11A: A gene that switches off fetal hemoglobin after birth. Gene therapies for sickle cell disease target BCL11A to switch fetal hemoglobin back on.
BCMA: B-cell maturation antigen, a protein found on the surface of nearly all myeloma cells. CAR T cell therapies and bispecific antibodies use it as their target.
belumosudil: A ROCK2 inhibitor (brand name Rezurock) FDA-approved for chronic GVHD after at least two prior lines of therapy. Works by rebalancing the immune response.
benign: Not cancerous and not expected to spread. A benign tumor grows slowly and does not invade other tissues.
benign variant: A genetic change confirmed to be harmless and not associated with disease.
benralizumab: A monoclonal antibody (brand name Fasenra) that targets the IL-5 receptor alpha and directly depletes eosinophils. Used in severe eosinophilic asthma and EGPA.
BEQVEZ: Brand name for fidanacogene elaparvovec, a one-time gene therapy approved for hemophilia B that enables the body to produce its own clotting factor.
beta-2-microglobulin: A small protein measured in the blood to stage multiple myeloma; higher levels mean more myeloma cells and, usually, a harder-to-treat disease.
beta-blocker: A long-used class of heart drugs (such as metoprolol, propranolol) that slow the heart rate and reduce the force of contraction. Usually the first medication tried in obstructive HCM.
bevacizumab: An antibody (brand Avastin) that blocks VEGF, starving tumors of new blood vessels. Used across several cancers.
bezuclastinib: Cogent Biosciences' pill that blocks KIT mutations that older drugs miss. Combined with sunitinib, it is under FDA priority review for GIST after imatinib, with a decision due November 30, 2026.
biallelic: Affecting both copies of a gene, one inherited from each parent. Recessive genetic diseases like Wilson disease require biallelic mutations.
bilateral: Affecting both sides of the body.
biliary dyskinesia: A condition in which the gallbladder does not empty properly, causing pain after meals even without gallstones.
bilirubin: A substance made when red blood cells break down. High levels can indicate liver problems.
bimekizumab: An antibody (brand Bimzelx) that blocks both IL-17A and IL-17F, approved for psoriasis, psoriatic arthritis, axial spondyloarthritis, and hidradenitis suppurativa.
bioavailability: The proportion of a drug that enters the bloodstream and reaches its target. Oral drugs typically have lower bioavailability than IV drugs.
biochemical control: Achievement of target lab values (like normal IGF-1 in acromegaly or reduced proteinuria in kidney disease) indicating a treatment is adequately controlling the disease.
biofilm: A protective community of microorganisms enclosed in a self-produced matrix that shields them from antibiotics and immune attack.
biologic: A drug made from living cells or organisms, such as antibodies or proteins.
biomarker: A measurable substance in your body that indicates a disease state or response to treatment.
biopsy: A small tissue sample taken from your body for lab testing.
biopsy-proven: Confirmed by laboratory examination of a tissue sample taken from the body.
biosimilar: A near-identical version of an approved biologic drug, allowed once the original's exclusivity ends. Comparable to a generic, but for complex biologic medicines.
Birmingham Vasculitis Activity Score: A clinical tool (BVAS) used to measure disease activity in systemic vasculitis including EGPA. Scores range from 0 (remission) to 63 (severe).
biventricular: Involving both the left and right ventricles (lower chambers) of the heart.
BLA: Biologics License Application: the formal request a company files asking the FDA to approve a biologic drug, such as an antibody or a gene therapy, for marketing.
black box warning: The FDA's most serious type of label warning, printed inside a black border at the top of the prescribing information, flagging a risk of serious injury or death.
blood-brain barrier: A protective layer of tightly packed cells lining blood vessels in the brain that prevents most substances in the blood from entering the brain.
BMS-986020: Bristol Myers Squibb's first LPA1 drug for IPF. It slowed lung decline in a Phase 2 trial but damaged the liver and gallbladder, and development stopped after 2018.
bone dysostosis: Skeletal deformities caused by abnormal bone development, resulting in characteristic body shape changes and joint stiffness.
bone marrow: The spongy tissue inside bones where blood cells (red cells, white cells, and platelets) are produced.
bone marrow biopsy: A procedure where a small sample of bone marrow is removed (usually from the hip bone) and examined to diagnose blood and bone marrow diseases.
bone marrow infiltration: Accumulation of abnormal cells (like mast cells) in the bone marrow, potentially disrupting normal blood cell production.
bone marrow suppression: A drug side effect where the bone marrow makes fewer blood cells than normal, leading to anemia, bleeding risk, or infection risk. Penicillamine can cause this in Wilson disease.
bone-spicule pigmentation: Dark pigment deposits in the retina with a bone-like pattern, a characteristic finding on eye exam in retinitis pigmentosa.
Borrelia: A genus of spiral-shaped bacteria (spirochetes) that cause Lyme disease and related tick-borne infections.
bovine: From cattle. On a drug label, a bovine-derived ingredient came from cow tissue.
boxed warning: The FDA's strongest safety warning on a drug label, framed in a black box at the top. It flags a serious or life-threatening risk that doctors and patients should weigh before use.
bradycardia: An abnormally slow heart rate.
bradykinesia: Slowness of movement, characteristic of Parkinson disease.
bradykinin: A protein that dilates blood vessels and increases their permeability, playing a role in inflammation, pain, and some types of swelling attacks.
brainstem: The lower part of the brain connecting to the spinal cord that controls essential functions like breathing, heart rate, and swallowing.
BRCA1: A DNA-repair gene. Inherited BRCA1 mutations raise the lifetime risk of breast, ovarian, and certain other cancers.
BRCA2: A DNA-repair gene. Inherited BRCA2 mutations raise the lifetime risk of breast, ovarian, prostate, and pancreatic cancers.
breakthrough therapy: An FDA designation for drugs showing substantial early improvement over existing treatments for serious conditions. It brings intensive FDA guidance and eligibility for faster review.
breakthrough therapy designation: An FDA designation that speeds development and review of drugs showing substantial improvement over existing treatments.
brepocitinib: An oral drug (brand Lisraya) that blocks TYK2 and JAK1 inflammation signals, the first oral targeted therapy approved for dermatomyositis.
Brineura: Brand name for cerliponase alfa, the only FDA-approved treatment for CLN2 Batten disease, given by infusion into the brain.
bronchiectasis: Permanent widening and scarring of the airways, causing mucus buildup and frequent lung infections.
bronchiolitis: Inflammation of the smallest airways of the lungs. The scarring form, bronchiolitis obliterans, is a serious complication of lung disease and graft-versus-host disease.
bronchiolitis obliterans: Scarring and narrowing of the small airways in the lungs, a serious complication of chronic GVHD that causes progressive shortness of breath.
bronchitis: Inflammation of the airways that carry air into the lungs, usually from an infection, causing a lasting cough.
bronchoscopy: An endoscopy procedure that examines the airways in the lungs.
budesonide: A corticosteroid prescribed for decades in asthma inhalers and for Crohn's disease. In IgA nephropathy it comes as a delayed-release capsule sold as Tarpeyo, built to open in the lower small intestine so it acts on the immune tissue making the faulty antibodies instead of dosing the whole body with steroid.
bullous: Relating to or characterized by large fluid-filled blisters (bullae) on the skin or mucous membranes.
burosumab: An antibody (brand Crysvita) that blocks FGF23, restoring phosphate balance in X-linked hypophosphatemia.
BVAS: Birmingham Vasculitis Activity Score — a standardized tool measuring disease activity in vasculitis conditions like EGPA and GPA.
bypassing agent: A clotting product used to stop bleeds in people whose inhibitors make factor replacement useless. The 2 in use are activated prothrombin complex concentrate (aPCC, sold as FEIBA) and recombinant factor VIIa.

C

C3 glomerulopathy: A rare kidney disease caused by uncontrolled activation of the complement system, leading to C3 protein deposits in the glomeruli and progressive kidney damage.
C3 nephritic factor: An autoantibody that stabilizes the C3 convertase enzyme, causing sustained complement activation. Found in many patients with C3 glomerulopathy.
C3G: C3 Glomerulopathy — a rare complement-mediated kidney disease where C3 protein accumulates in the glomeruli, causing inflammation and progressive kidney damage.
C3NeF: C3 Nephritic Factor — an autoantibody that prevents normal regulation of the complement system, driving sustained C3 activation and kidney damage in C3G.
C5: A protein near the end of the complement cascade, the part of your immune system that amplifies attacks on invaders. Blocking C5 shuts down the final damaging step, which is how Ultomiris works.
cabozantinib: A multi-kinase inhibitor (brand name Cometriq/Cabometyx) approved for advanced medullary thyroid cancer and other cancers.
cachexia: Severe muscle and weight loss driven by an underlying illness such as cancer or heart failure, which eating more does not reverse.
café-au-lait: Flat, light-brown birthmarks on the skin. Six or more café-au-lait spots larger than 5 mm in children is a diagnostic criterion for neurofibromatosis type 1.
cafe-au-lait: Flat, light-brown birthmarks on the skin. Six or more café-au-lait spots is a hallmark of neurofibromatosis type 1.
CAG repeat: A specific trinucleotide repeat in DNA. Expansion of CAG repeats causes Huntington disease and other disorders.
calcinosis: Abnormal deposits of calcium in soft tissues like skin, muscles, or organs where calcium does not normally accumulate.
calcitonin: A hormone produced by thyroid C cells. Elevated calcitonin in blood is a key marker for diagnosing and monitoring medullary thyroid cancer.
camrelizumab: An immunotherapy antibody (a PD-1 inhibitor) that helps the immune system recognize and attack cancer cells. Studied in combination with rivoceranib for liver cancer.
canakinumab: A monoclonal antibody (brand name Ilaris) that blocks interleukin-1β. FDA-approved for sJIA and other autoinflammatory conditions.
caprine: From goats.
CAR-T: A treatment that takes a patient's own T cells, engineers them in a lab to find a target such as CD19, and infuses them back. In autoimmune diseases like myasthenia gravis it is being tested as a way to reset the immune system by clearing out B cells.
carcinoembryonic antigen: A protein (CEA) used as a tumor marker. In medullary thyroid cancer, rising levels can signal disease progression.
carcinoma: A type of cancer that starts in cells lining the inner or outer surfaces of the body (epithelial cells). The most common category of cancer.
carcinomatosis: Widespread spread of cancer throughout a body cavity (usually the abdomen), with many tumor deposits on organ surfaces.
cardiac: Relating to the heart.
cardiac hypertrophy: Abnormal thickening of the heart muscle, which can reduce the heart's ability to pump blood efficiently.
cardiac monitoring: Regular testing of heart function — including ECG, echocardiogram, and sometimes Holter monitoring — to detect changes in heart rhythm or structure.
cardiac MRI: An MRI scan of the heart that shows muscle thickness, function and scarring. In ATTR amyloidosis, a typical contrast pattern can suggest amyloid in the heart wall.
cardiac myosin inhibitor: A drug class (mavacamten/Camzyos, aficamten) that quiets the heart muscle's contracting protein so the thickened heart squeezes less forcefully and blood can leave the chamber more easily.
cardiomegaly: An enlarged heart, which can result from high blood pressure, valve disease, or other conditions that make the heart work harder.
cardiomyopathy: Disease of the heart muscle that makes it difficult for the heart to pump blood.
cardiotoxic: Capable of damaging the heart muscle. Some cancer treatments carry cardiotoxicity risk and require heart monitoring.
cardiovascular: Relating to the heart and blood vessels.
carpal tunnel syndrome: Numbness and tingling in the hand from a squeezed nerve at the wrist. Common on its own, but in both hands it can be an early sign of ATTR amyloidosis, often years before the heart is affected.
carrier screening: Testing to determine if someone carries a genetic mutation for a disease but is not affected.
CAS: Clinical Activity Score, the 7-point inflammation checklist for thyroid eye disease; 3 or more means active disease.
cascade screening: Systematic genetic testing of family members of a patient with a genetic disease.
Casgevy: Brand name for exagamglogene autotemcel, the first CRISPR gene-editing therapy approved by the FDA, for sickle cell disease and beta thalassemia.
cataract: Clouding of the eye's natural lens, causing blurred or dim vision. Cataracts develop gradually and are treatable with surgery.
catastrophic coverage: The last stage of Medicare Part D. Once your out-of-pocket drug costs reach the yearly cap, you pay nothing for covered Part D drugs for the rest of the year.
catecholamine: Hormones (dopamine, adrenaline, noradrenaline) released by the adrenal glands that increase heart rate and blood pressure during stress.
catheterization: Inserting a thin tube (catheter) into a blood vessel or body cavity for diagnostic testing or treatment.
CD19: A marker protein on the surface of B cells. CAR T and antibody therapies target CD19 to clear out disease-causing B cells.
CD20: A marker protein on the surface of B cells. Antibodies that target CD20 (such as ublituximab, ocrelizumab, and rituximab) remove B cells to calm autoimmune disease.
CD3: A protein on T cells that activates them. Some therapies grab CD3 on one side and a target cell on the other, steering the immune system's attack.
CD38: A protein found in large amounts on the surface of plasma cells, including the abnormal ones in multiple myeloma and AL amyloidosis. Antibodies such as Darzalex attach to CD38 and mark those cells for the immune system to destroy.
CDKL5: A gene essential for early brain development. CDKL5 deficiency disorder causes seizures beginning in infancy and severe developmental delay.
CEA: Carcinoembryonic Antigen, a tumor marker protein. In medullary thyroid cancer, rising CEA can signal disease progression.
cell adhesion: The process by which cells attach to each other or to surrounding structures, essential for holding tissues together.
Cell and Gene Therapy Access Model: A CMS program in which the federal government negotiates outcomes-based terms for sickle cell gene therapies (Casgevy and Lyfgenia) on behalf of participating state Medicaid programs.
cell therapy: Medical treatment using living cells (often modified) introduced into a patient's body to treat disease or rebuild damaged tissue.
cell-based assay: A blood test that detects an antibody by exposing it to living cells that carry the target protein. For the MOG antibody it is the only type of test considered reliable enough to make the diagnosis.
CELLECTRA: INOVIO's handheld device that delivers the electrical pulse after an INO-3107 injection, helping the DNA enter muscle cells.
cellulitis: A bacterial skin infection causing redness, swelling, warmth, and pain in the affected area.
CELMoD: Cereblon E3 ligase modulator, a newer class of oral myeloma drug related to lenalidomide that works by tagging 2 proteins myeloma cells rely on, Ikaros and Aiolos, for destruction. Mezigdomide is the first under FDA review.
cemdisiran: An investigational siRNA from Regeneron that tells the liver to make less complement protein C5, injected under the skin every 12 weeks. The FDA is reviewing it for adults with AChR antibody-positive generalized myasthenia gravis, with a decision expected in November 2026.
central scotoma: A blind spot or area of reduced vision in the center of the visual field, typical of Stargardt disease and macular degeneration.
cerebellar: Relating to the cerebellum, the part of the brain that controls balance and coordination.
cerebellar ataxia: Loss of coordination and balance caused by damage to the cerebellum, the brain region that controls movement.
cerebellum: The part of the brain at the back of the skull that coordinates movement, balance, and fine motor control.
cerebral cortex: The outer layer of the brain responsible for thinking, memory, language, and conscious awareness.
cerebral edema: Dangerous swelling of the brain tissue that can occur with severe metabolic disease.
cerebrospinal fluid: The clear fluid that surrounds the brain and spinal cord, cushioning and protecting them.
cerliponase alfa: An enzyme replacement therapy (brand name Brineura) delivered directly into the brain to treat CLN2 Batten disease.
ceruloplasmin: A protein in the blood that carries copper. Low ceruloplasmin levels are a key diagnostic marker for Wilson disease.
cetuximab: A monoclonal antibody used in colorectal cancer (brand name Erbitux). Cetuximab is produced in mouse cell lines and carries alpha-gal residues; the 2008 anaphylactic reactions to cetuximab were the diagnostic clue that led to AGS being recognized.
CFTR: The gene (and protein) that controls salt and water movement across cell surfaces. Faulty CFTR causes the thick, sticky mucus of cystic fibrosis.
CGP: Comprehensive Genomic Profiling, a broad genetic test that screens tumor tissue for many cancer-driving mutations simultaneously.
cGVHD: Chronic Graft-Versus-Host Disease — a long-term complication after allogeneic stem cell or bone marrow transplant where donor immune cells attack the recipient's organs.
Charcot-Marie-Tooth: An inherited disease that damages the peripheral nerves (the nerves connecting the spinal cord to muscles, skin, and limbs), causing slow progressive weakness in the lower legs and hands plus loss of sensation. The most common inherited neurological disorder.
chelation: A treatment using drugs that bind to excess metals (like copper or iron) in the body and help remove them through urine.
chelation therapy: Treatment using drugs that bind to and remove toxic metals from the body.
chelator: A drug that binds tightly to a metal (like copper or iron) so the body can flush it out, usually through urine. Wilson disease chelators include penicillamine, trientine, and tetrathiomolybdate.
chemoradiation: A treatment combining chemotherapy drugs with radiation therapy, often used together to increase effectiveness against cancer.
chemotherapy: Cancer treatment using drugs that kill rapidly dividing cells.
cherry-red spot: A red spot visible on the retina during eye examination, seen in certain lysosomal storage diseases.
CHMP: The Committee for Medicinal Products for Human Use, the European Medicines Agency panel that recommends whether a drug should be approved in the European Union. The European Commission makes the final decision, usually within about 2 months.
cholangiocarcinoma: Cancer of the bile ducts, the tubes that carry bile from the liver to the intestine. A rare cancer, often driven by gene changes such as FGFR2 fusions.
cholangitis: Inflammation or infection of the bile ducts that carry bile from the liver to the gut. It can cause fever, belly pain and yellowing of the skin.
cholecystitis: Inflammation of the gallbladder, usually causing pain under the right ribs, fever and nausea. Three drug-related cases stopped the BMS-986020 trial in IPF.
cholestasis: A condition where bile flow from the liver is reduced or blocked, causing buildup of bile acids that can damage the liver.
chondrosarcoma: A rare cancer of cartilage cells, most often in the pelvis, thigh, or shoulder. It usually resists chemotherapy and radiation, so surgery has been the main treatment.
chorea: Involuntary, irregular, jerky movements that characterize Huntington disease.
chromatin: The tightly packaged combination of DNA and proteins that makes up chromosomes inside the cell nucleus.
chromoendoscopy: An endoscopy technique where dye is applied to the lining of the digestive tract to highlight abnormal areas more clearly.
chromosomal inversion: A type of mutation where a segment of a chromosome is reversed end to end, which can disrupt genes at the breakpoints.
chronic: Long-lasting or recurring over a long period of time.
Churg-Strauss: The former name for eosinophilic granulomatosis with polyangiitis (EGPA), a rare vasculitis with asthma and eosinophilia.
chylomicronemia: Extremely high triglyceride (blood fat) levels that can trigger dangerous pancreatitis, seen in familial chylomicronemia syndrome.
cidofovir: An antiviral drug injected directly into the airway after RRP surgery in some patients, with mixed results.
CIDP: Chronic inflammatory demyelinating polyneuropathy, a rare autoimmune disease in which the immune system strips the insulation from peripheral nerves, causing weakness and numbness.
cipaglucosidase alfa: A recombinant acid alpha-glucosidase enzyme (brand name Pombiliti) designed with high mannose-6-phosphate content for better cellular uptake in Pompe disease treatment.
cirrhosis: Severe scarring of the liver from long-term damage, impairing the liver's ability to function and filter blood.
CK level: Creatine Kinase level, a blood test measuring muscle enzyme leakage. Very high CK indicates ongoing muscle damage.
cladribine: An oral drug (brand Mavenclad) approved for relapsing multiple sclerosis that lowers B and T cells in short treatment courses. It is in Phase 3 testing for generalized myasthenia gravis.
claseprubart: An investigational antibody from Dianthus Therapeutics that blocks C1s, an early step in the complement system, in Phase 3 testing for generalized myasthenia gravis.
ClC-1: Chloride ion channel 1, a protein on muscle fibers that normally limits how excitable the muscle is. Blocking ClC-1 makes muscle fibers more responsive to weak nerve signals, the basis of ignaseclant in CMT.
clinical activity score: A 7-point checklist doctors use to grade how inflamed thyroid eye disease is right now: pain, redness and swelling of the lids and eye each score a point. A score of 3 or more means active disease, and a drop of 2 points is the usual sign a treatment is working.
clinical trial: A research study testing the safety and effectiveness of new treatments in patients.
clinically significant: Important enough to affect your health or treatment decisions.
CLN1: A form of Batten disease (neuronal ceroid lipofuscinosis type 1), a rare inherited brain disorder of childhood caused by a missing recycling enzyme.
CLN2: A form of Batten disease (neuronal ceroid lipofuscinosis type 2) caused by deficiency of the TPP1 enzyme, treatable with enzyme replacement.
CLN2 disease: A form of Batten disease caused by TPP1 enzyme deficiency. The only type with an FDA-approved enzyme replacement therapy (Brineura).
clonal: Relating to a group of identical cells all descended from a single parent cell. Clonal expansion means abnormal multiplication of one cell line.
clonus: Rhythmic muscle contractions and relaxations triggered by stretching the muscle.
CMT Neuropathy Score: A clinical measurement scale that combines patient-reported symptoms, physical exam findings, and nerve conduction results to track CMT severity over time. Used as an endpoint in many CMT clinical trials.
CMT-SORD: A subtype of Charcot-Marie-Tooth disease caused by mutations in the SORD gene, which leads to buildup of a toxic substance called sorbitol that damages nerves. Govorestat (AT-007) is in clinical trials for this subtype.
CMT1A: The most common subtype of Charcot-Marie-Tooth disease, caused by an extra copy of the PMP22 gene. Accounts for about 60% of CMT cases. Damages the myelin coating around nerves.
CMT1B: A demyelinating subtype of Charcot-Marie-Tooth disease caused by mutations in the MPZ (myelin protein zero) gene.
CMT2A: An axonal subtype of Charcot-Marie-Tooth disease caused by mutations in the MFN2 gene. Damages the nerve fibers themselves rather than their myelin coating.
CMT2S: A recessive subtype of Charcot-Marie-Tooth disease caused by mutations in the IGHMBP2 gene. AAV9-based gene replacement therapy is being tested in early-phase trials for this subtype.
CMT4: A group of recessive (both gene copies affected) Charcot-Marie-Tooth subtypes that tend to be more severe and to present earlier in life.
CNP: C-type natriuretic peptide, a hormone the body makes that pushes back against the overactive FGFR3 growth brake in achondroplasia. Voxzogo and Yuviwel are both forms of it.
CNS: Central Nervous System — the brain and spinal cord.
CNS-penetrant: Able to cross the blood-brain barrier and reach tumors or disease sites within the brain and spinal cord.
coagulation: The process by which blood forms clots to stop bleeding. Disorders of coagulation can cause excessive bleeding or clotting.
coagulopathy: A disorder that impairs the blood's ability to clot properly, leading to either excessive bleeding or abnormal clotting.
coarse facial feature: Thickened or prominent facial features, seen in some lysosomal storage disorders.
cochlear: Relating to the cochlea, the snail-shaped structure in the inner ear that converts sound vibrations into nerve signals.
codon mutation: A mutation at a specific position in a gene. In RET-related cancers, the codon location helps predict cancer aggressiveness and guides treatment timing.
cofactor: A molecule that helps an enzyme work properly. Some genetic diseases can be partially treated by providing extra cofactor.
cognitive decline: Gradual loss of mental abilities including memory, thinking, and problem-solving.
cohort: A group of patients in a study who share a common characteristic, such as receiving the same treatment or having the same disease stage.
coinfection: Simultaneous infection with two or more pathogens, common in tick-borne diseases because a single tick can carry multiple organisms.
coinsurance: Your share of a covered cost as a percentage of the price, such as 20% or 25%, rather than a flat copay.
COL3A1: The gene for type III collagen, which gives strength to blood vessels and organs. COL3A1 mutations cause vascular Ehlers-Danlos syndrome.
COL7A1: The gene for collagen VII, the protein that anchors the outer skin to the layers beneath. Mutations cause dystrophic epidermolysis bullosa, where skin blisters and tears from minor friction.
colitis: Inflammation of the colon (large intestine), causing diarrhea, abdominal pain, and sometimes bleeding.
collagen: A structural protein that provides strength and flexibility to skin, bones, tendons, and other connective tissues.
colonoscopy: An endoscopy procedure that examines the colon and lower intestines.
combination therapy: Treatment using multiple drugs or therapies together.
COMET trial: A Phase III clinical trial comparing Nexviazyme (avalglucosidase alfa) head-to-head against Lumizyme (alglucosidase alfa) in late-onset Pompe disease.
comorbid: Another health condition you have in addition to the one being studied.
comorbidity: An additional disease or condition existing alongside the primary disease, which can complicate treatment and outcomes.
companion diagnostic: A test used to determine whether a patient's disease has a specific marker that makes them likely to benefit from a particular treatment.
compassionate use: A program allowing patients with serious illnesses to use investigational drugs outside of clinical trials when no approved alternatives exist.
complement: A group of immune system proteins in the blood that work together to fight infections but can cause damage when overactive.
complement activation: The triggering of complement blood proteins, which amplifies the immune response. Uncontrolled activation can damage healthy tissues.
complement C5: A protein in the complement immune system. Blocking C5 prevents immune-mediated destruction of cells in diseases like myasthenia gravis and PNH.
complement cascade: A chain reaction of immune proteins in the blood that can destroy cells. In autoimmune diseases, it can mistakenly attack the body's own tissue.
complement factor B: A protein in the alternative complement pathway. Factor B inhibitors like iptacopan block this protein to reduce complement overactivation.
complement factor H: A regulatory protein that controls the alternative complement pathway. Mutations or autoantibodies against factor H can cause C3 glomerulopathy and aHUS.
complement inhibitor: A drug that blocks part of the complement system, a group of proteins in your blood that can amplify inflammation and kidney damage.
complement pathway: A chain reaction of immune proteins in the blood that, when overactive, can worsen inflammation and tissue damage in the kidneys.
complement system: A group of blood proteins that work with antibodies to destroy bacteria and clear infections. Overactivation can damage the body's own tissues.
complete remission: In kidney diseases such as membranous nephropathy, a drop in urine protein to near-normal levels with stable kidney function, meaning the disease is essentially quiet.
complete response: All signs of the disease have disappeared after treatment, though it may not mean the disease is cured.
complete response letter: A formal FDA letter issued when the agency decides it cannot approve a drug application in its current form, often citing needed additional data or manufacturing issues.
compound heterozygote: A person with two different mutations in the same gene (one from each parent).
compound heterozygous: Carrying two different mutations in the same gene, one inherited from each parent. Can cause disease even though neither parent is affected.
comprehensive care: A coordinated approach to treatment that addresses all aspects of a patient's condition through multiple specialists.
comprehensive genomic profiling: Broad genetic testing of tumor tissue that screens for hundreds of cancer-driving mutations at once, including NTRK and RET fusions.
concizumab: The generic name of Alhemo, a daily anti-TFPI antibody injected under the skin to prevent bleeds in hemophilia A or B from age 12, with or without inhibitors.
concomitant: Taken or happening at the same time as another treatment.
conduction system disease: A problem with the heart's electrical wiring that disrupts the normal rhythm of heartbeats, potentially causing slow or irregular heart rhythms.
cone-rod dystrophy: A variant of retinal degeneration where central vision and color vision are affected earlier than night vision, unlike typical RP.
confidence interval: A range of values that likely contains the true treatment effect. A narrower interval means more certainty about the result.
confirmatory evidence: In FDA language, the extra support that lets 1 well-controlled trial stand in for the usual 2: a second trial that leaned the same way, strong results on related measures, or data from the same drug in a related disease.
confirmatory trial: A follow-up clinical trial required after accelerated approval to verify that a drug provides real clinical benefit.
confirmed disability progression: A worsening on the EDSS disability scale that is still present when re-measured 12 or 24 weeks later, used in MS trials to separate lasting decline from a passing relapse.
congenital: Present from birth, whether caused by genetic factors, events during pregnancy, or birth complications.
conjunctival: Relating to the conjunctiva, the thin transparent membrane covering the white of the eye and inner eyelids.
connective tissue: Tissue that provides structural support throughout the body, including collagen and elastin.
consolidation therapy: Treatment given after initial remission to eliminate any remaining disease and strengthen the response.
constitutively active: A protein that is permanently switched on, driving uncontrolled cell growth without needing normal activation signals.
continuation of benefits: Keeping an existing treatment covered while an appeal is pending. Common in Medicaid when the appeal is filed on time; you usually have to ask for it in writing.
contract research organization: A company hired by pharmaceutical firms to conduct clinical trials and provide research services.
contracture: Permanent tightening of muscles, tendons, or joints that limits movement, often caused by prolonged immobility or muscle disease.
contraindicated: Should not be used in that situation, because the risk outweighs any benefit.
contraindication: A medical reason why a particular treatment should not be used.
control group: Patients in a study who receive the standard treatment or placebo instead of the experimental treatment, serving as the comparison.
copay: The share of a medication's cost you pay out of pocket. Manufacturer copay cards (commercial insurance only) and independent foundations can cover most or all of it for rare disease drugs.
copper histidine: A form of copper supplement (copper-histidine complex) used as replacement therapy in Menkes disease. Most effective when started in the neonatal period before irreversible brain damage occurs.
copper-transporting ATPase: A protein that moves copper across cell membranes. Defects in this protein (encoded by ATP7A) cause Menkes disease.
corneal: The clear dome-shaped front surface of the eye that focuses incoming light. Damage or clouding impairs vision.
corneal clouding: Progressive clouding of the clear front part of the eye from material deposits, causing blurred vision. Common in MPS I and other storage disorders.
corneal verticillata: A whorl-shaped pattern in the cornea of the eye, visible with a slit lamp exam. A hallmark finding in Fabry disease that does not affect vision.
cortical: Relating to the cortex, the outer layer of the brain.
corticosteroid: A class of anti-inflammatory drugs that mimic the hormone cortisol, used to reduce inflammation and suppress immune activity. Long-term use can cause significant side effects.
cortisol: A stress hormone produced by the adrenal glands that regulates metabolism, immune function, and the body's response to stress.
coverage determination: A Medicare Part D plan's first decision on whether it will cover a drug, including exception requests. The plan must answer within 72 hours, or 24 hours if expedited.
CPS1: A gene for the first enzyme of the urea cycle. CPS1 deficiency causes dangerous ammonia buildup, usually appearing in newborns.
cranial: Relating to the skull or the cranial nerves that emerge directly from the brain to control face, eye, and head functions.
crazy paving: A pattern on a chest CT scan of hazy patches crossed by fine lines, which looks like irregular paving stones. It is typical of pulmonary alveolar proteinosis but can also appear in other lung conditions.
creatine kinase: A muscle enzyme released into the blood when muscle fibers are damaged, so a raised level signals ongoing muscle breakdown. Extremely high levels are a hallmark of Duchenne muscular dystrophy, and elevations are also seen in Pompe disease.
creatinine: A waste product filtered by the kidneys. Blood creatinine is the standard lab used to estimate how well the kidneys are working.
creatinine clearance: A test measuring how well your kidneys filter waste from blood.
CRISPR: A gene-editing technology that lets scientists precisely cut and modify DNA sequences. Used therapeutically to correct disease-causing mutations.
crizanlizumab: An antibody (brand Adakveo) that blocks P-selectin to help prevent painful vaso-occlusive crises in sickle cell disease.
CRL: Complete Response Letter. A formal FDA letter issued when the agency decides it cannot approve a drug application in its current form, often citing needed additional data or manufacturing issues.
CRO: Contract Research Organization — a company providing outsourced research and clinical trial services.
croscarmellose sodium: A plant-derived (cellulose) ingredient that helps tablets break apart in the stomach. Not animal-derived.
crospovidone: A synthetic polymer used to bind tablets or help them dissolve. Not animal-derived.
crossover trial: A study where participants receive multiple treatments in sequence, each serving as their own control.
cryoglobulinemia: A condition where abnormal proteins in the blood clump together in cold temperatures, blocking small blood vessels and causing inflammation.
cryotherapy: Treatment that uses extreme cold to freeze and destroy abnormal tissue, including some cancers and skin lesions.
cryptogenic: Of hidden or unknown origin. When a condition is cryptogenic, doctors cannot determine its underlying cause.
CSF1R: A receptor that the brain's immune cells (microglia) depend on. CSF1R mutations cause ALSP, a rare adult-onset brain disease.
CT scan: Computed Tomography scan — an X-ray based imaging test that creates cross-sectional images.
CTCAE: Common Terminology Criteria for Adverse Events — the standard scale doctors use to grade side effects (1=mild, 5=fatal).
ctDNA: Circulating tumor DNA: fragments of a tumor's DNA in the blood. A blood test for it (a liquid biopsy) can show which resistance mutations a cancer has developed.
cyanosis: A bluish discoloration of the skin and mucous membranes caused by insufficient oxygen in the blood.
cyclodextrin: A ring-shaped sugar molecule that can pull cholesterol out of cells. Adrabetadex is a cyclodextrin given into the spinal fluid for Niemann-Pick disease type C.
CYP2C19: A liver enzyme that breaks down certain drugs. People inherit fast or slow versions of it, and some other medicines block it, which changes how much of a drug like mavacamten stays in the blood.
cystinosis: An inherited disease in which the amino acid cystine builds up inside cells and forms crystals, damaging the kidneys, eyes and other organs.
cystitis: Inflammation of the bladder, usually caused by a bacterial infection, causing frequent and painful urination.
cytokine: Small signaling proteins released by immune cells that coordinate the body's response to infection, inflammation, and disease.
cytologically: Relating to the examination of individual cells, usually from a fluid sample.
cytopenia: Lower-than-normal counts of one or more types of blood cells (red cells, white cells, or platelets), often a sign of bone marrow suppression.
cytotoxic: Toxic to cells. Cytotoxic drugs (chemotherapy) work by killing rapidly dividing cancer cells.

D

DailyMed: The National Library of Medicine's official, always-current archive of FDA drug labels.
daratumumab: An antibody drug (brands Darzalex and Darzalex Faspro) that binds CD38, a protein on myeloma cells, and marks them for destruction by the immune system.
DDD: Dense Deposit Disease — a subtype of C3 glomerulopathy with characteristic dense deposits in the kidney's glomerular basement membrane.
de novo: Newly developed, not arising from a previous condition.
de novo mutation: A genetic mutation that appears for the first time in a person, not inherited from parents.
de-coppered: A clinical state in Wilson disease where a patient's copper burden has been brought down to safe levels through treatment, allowing transition to maintenance therapy.
Debio 4126: Debiopharm's long-acting octreotide injection given every 3 months instead of monthly, in the Phase 3 OXTEND-03 trial for acromegaly.
debulking: Surgery to remove as much of a tumor as possible when complete removal is not achievable, often followed by additional treatment.
defibrotide: A drug (brand Defitelio) made from pig intestinal tissue, used to treat a severe liver complication, veno-occlusive disease, after a stem cell transplant.
degranulation: The release of chemical contents (like histamine) stored inside immune cells, particularly mast cells, often triggering an allergic or inflammatory response.
delandistrogene moxeparvovec: A gene therapy (brand name Elevidys) that uses an AAV vector to deliver a micro-dystrophin gene to muscle cells in Duchenne muscular dystrophy.
delayed allergic reaction: An allergic reaction that occurs hours after exposure rather than within minutes, characteristic of Alpha-Gal Syndrome (usually 2 to 6 hours after eating red meat or dairy, according to the CDC).
deletion: A type of mutation where a section of DNA is missing. Can range from a single letter to an entire gene or chromosome region.
dementia: A group of conditions causing progressive decline in memory, thinking, and the ability to perform everyday activities.
demyelinating: Causing loss of myelin, the fatty insulation that wraps around nerve fibers and helps electrical signals travel quickly. CMT1 and multiple sclerosis are examples of demyelinating conditions.
demyelination: Loss of myelin insulation around nerve fibers, which disrupts nerve signal transmission.
dendritic cell vaccine: A vaccine made from a patient's own immune messenger cells, trained in the lab to show the immune system what to attack.
denecimig: Novo Nordisk's factor VIIIa mimetic antibody (Mim8, brand name Frehemgo in Europe) for hemophilia A, designed for monthly, every-2-week or weekly injection under the skin and under FDA review as of September 30, 2026.
dense deposit disease: A subtype of C3 glomerulopathy (formerly MPGN type II) characterized by dense ribbon-like deposits within the glomerular basement membrane.
deposition: The abnormal buildup or accumulation of a substance (such as proteins, antibodies, or minerals) in body tissues where it does not belong.
depot: A slow-release reservoir of drug placed by injection into muscle or under the skin, designed to release medicine steadily over weeks.
depot injection: An injection that deposits a slow-dissolving reservoir of drug into muscle or under the skin, releasing it over weeks. This is how long-acting octreotide and lanreotide deliver a month of medicine in 1 shot.
dermal-epidermal junction: The boundary where the outer skin layer (epidermis) meets the inner layer (dermis), held together by specialized anchoring proteins.
dermatan sulfate: A type of glycosaminoglycan (complex sugar chain) found in skin, blood vessels, and heart valves. Accumulates in MPS I.
dermatitis: Inflammation of the skin causing itching, redness, and sometimes blistering or flaking.
dermatomyositis: An inflammatory disease causing muscle weakness along with a distinctive skin rash on the face, knuckles, and other areas.
desmoplastic: Characterized by dense fibrous (scar-like) tissue growth, often seen in aggressive tumors that stimulate the surrounding tissue to become fibrotic.
desmosomal: Relating to desmosomes, the protein structures that rivet neighboring cells together. Defects in desmosomal proteins can weaken heart muscle.
developmental delay: Slower than normal development of physical, cognitive, or social skills in children.
developmental quotient: A score comparing a child's developmental age to their actual age, with 100 meaning on track. The Fayuvi trial required a score of at least 60 in children older than 2.
developmental regression: Loss of previously acquired skills and abilities, as seen in Rett syndrome.
diagnosis: The process of identifying a disease or condition based on symptoms, tests, and medical history.
dialysis: A procedure that filters waste from the blood when the kidneys cannot.
diastolic: Relating to the phase when the heart muscle relaxes and fills with blood. Diastolic blood pressure is the bottom number in a reading.
differential diagnosis: The process of distinguishing a particular disease from others with similar symptoms by systematically comparing their features.
differentiation: How much cancer cells resemble normal cells. Well-differentiated cancers grow slowly; poorly differentiated cancers are more aggressive.
diflunisal: An older anti-inflammatory pain reliever that also stabilizes transthyretin. A 2013 trial showed it slowed nerve damage in hereditary ATTR amyloidosis, and some doctors use it off label.
dilated cardiomyopathy: A heart condition where the main pumping chamber becomes enlarged and weakened, reducing its ability to pump blood effectively.
dinutuximab: An antibody (brand Unituxin) that flags neuroblastoma cells for immune attack, used in high-risk childhood neuroblastoma.
diplopia: Double vision, seeing two images of a single object, caused by problems with eye muscles or nerves.
disease control rate: The share of patients whose tumors shrank or stopped growing on treatment, counting both responses and stable disease.
disease progression: The development and worsening of a disease over time.
disease-modifying: A treatment that slows or halts disease progression, rather than just relieving symptoms.
dispersible tablet: A tablet designed to be mixed into water and given by syringe or feeding tube instead of swallowed whole.
distal: Relating to the part of a limb farthest from the body center.
DLCO: Diffusing capacity of the lungs for carbon monoxide, a breathing test that measures how well gas crosses from the air sacs into the blood. It was the main measure in the molgramostim trial for pulmonary alveolar proteinosis.
DM1: Shorthand for myotonic dystrophy type 1, an inherited disease in which a repeating stretch of DNA in the DMPK gene produces toxic RNA that disrupts muscle and other organs.
DMD: Duchenne Muscular Dystrophy, a severe X-linked genetic disorder causing progressive muscle degeneration and weakness due to absence of dystrophin protein.
dopamine: A neurotransmitter involved in movement, mood, and motivation.
dopaminergic: Relating to dopamine, a brain chemical involved in movement, motivation, and reward. Dopaminergic neurons are affected in Parkinson's disease.
dorsal root ganglion: Clusters of nerve cells near the spinal cord that process sensory information.
dose escalation: Gradually increasing the amount of a drug given to patients to find the best balance between effectiveness and side effects.
dose-limiting toxicity: A side effect serious enough that the drug dose cannot be increased further without unacceptable harm to the patient.
dose.escalation: A trial design that starts on low drug doses and gradually increases to find the maximum tolerated dose.
double-blind: A trial where neither patients nor researchers know which treatment is being given.
DR5: Death receptor 5, a protein on the surface of some cancer cells that triggers cell death when activated. DR5 agonist antibodies are designed to switch it on in tumors.
driver mutation: The genetic change that is actually causing a cancer to grow, as opposed to the many harmless changes a tumor accumulates. Finding the driver is what makes a targeted drug possible.
drug-drug interaction: When one medication affects how another medication works, potentially increasing side effects or reducing effectiveness.
drug-resistant seizures: Seizures that don't respond adequately to multiple anti-seizure medications, common in progressive neurological conditions like Batten disease.
DSMB: Data Safety Monitoring Board: an independent group of clinicians and statisticians who review a trial's accumulating data while it runs and can recommend stopping it early for safety or overwhelming benefit.
DTx-1252: An siRNA therapy designed to lower PMP22 levels in CMT1A, delivered through DTx Pharma's FALCON conjugation platform. Acquired by Novartis through DTx Pharma in 2023.
Duchenne muscular dystrophy: A severe genetic muscle-wasting disease caused by mutations in the dystrophin gene. It primarily affects boys and causes progressive weakness starting in early childhood.
duodenum: The first and shortest section of the small intestine, where food from the stomach first meets digestive enzymes.
duplication: A mutation where a section of DNA is copied an extra time, potentially disrupting gene function.
duration of response: How long a treatment continues to work after the tumor first started shrinking.
dysarthria: Difficulty with speech clarity and articulation due to muscle weakness or nerve damage.
dysbiosis: An imbalance in the normal microbial community (microbiome) living in the gut, which can contribute to inflammation and disease.
dyskeratosis: Abnormal maturation of skin cells, seen as thickened or unusual skin patches and associated with certain genetic syndromes.
dyslipidemia: Abnormal levels of fats (cholesterol and triglycerides) in the blood, a major risk factor for cardiovascular disease.
dysostosis: Defective bone formation, often resulting in skeletal abnormalities present from birth.
dyspepsia: Persistent indigestion or discomfort in the upper abdomen, sometimes called an upset stomach.
dysphagia: Difficulty with swallowing, which can make eating and drinking challenging.
dysplasia: Abnormal development or growth of cells in a tissue, sometimes a precursor to cancer.
dyspnea: Shortness of breath.
dystonia: Involuntary muscle contractions that cause repetitive, twisting movements or abnormal postures.
dystrophin: A protein that holds muscle fibers together and keeps them stable during contraction. It is absent in Duchenne muscular dystrophy and partly working in the milder Becker muscular dystrophy.
dystrophy: Progressive weakness and degeneration of tissue, especially muscle. Used in conditions like muscular dystrophy.

E

ecallantide: An injectable drug (brand Kalbitor) that blocks kallikrein to treat acute hereditary angioedema attacks.
ecchymosis: A bruise, caused by blood leaking from damaged small blood vessels into the surrounding tissue beneath the skin.
echocardiogram: An ultrasound of the heart that creates detailed images of heart structure and function.
ECOG: A score (0-5) measuring how well you can do daily activities. 0 = fully active, 5 = deceased.
ectopia: The displacement of an organ or tissue from its normal position, such as an ectopic kidney or dislocated eye lens.
ectopia lentis: Dislocation of the lens in the eye, seen in homocystinuria and Marfan syndrome.
eculizumab: An antibody (brand Soliris) that blocks complement protein C5, used in PNH, aHUS, myasthenia gravis, and NMOSD.
eczema: A chronic skin condition causing dry, itchy, inflamed patches of skin that can crack and weep.
edema: Swelling caused by excess fluid trapped in body tissues.
EDSS: Expanded Disability Status Scale, the 0 to 10 score neurologists use to grade MS disability, where higher numbers mean more impairment.
EEG: Electroencephalogram, a test that measures electrical activity in the brain to help diagnose seizures and other neurological conditions.
efficacy: How well a treatment works at producing the desired result.
effusion: Abnormal buildup of fluid in a body cavity, such as around the lungs (pleural effusion) or heart (pericardial effusion).
efgartigimod: An FcRn-blocking therapy (brand Vyvgart) that quickly lowers harmful IgG antibodies, approved for CIDP and, since May 2026, for all adults with generalized myasthenia gravis whatever their antibody test shows.
eGFR: Estimated Glomerular Filtration Rate — measures kidney function.
eGFR slope: How fast kidney function changes each year, measured as the yearly change in eGFR. A slope close to zero means kidney function is holding steady.
EGPA: Eosinophilic Granulomatosis with Polyangiitis — a rare eosinophilic vasculitis (formerly Churg-Strauss syndrome) affecting small vessels, with asthma and eosinophilia.
Ehrlichia: A genus of bacteria transmitted by ticks that infect white blood cells, causing ehrlichiosis. Treated with doxycycline.
ehrlichiosis: A tick-borne bacterial infection that causes fever, headache, muscle aches, and nausea. Caused by Ehrlichia bacteria and treated with doxycycline.
EIA: Enzyme Immunoassay; a lab test that uses antibody-antigen reactions to detect infections or immune responses in blood.
ejection fraction: The percentage of blood pumped out of your heart with each beat. Normal is 55-70%.
Elaprase: Brand name for idursulfase, the FDA-approved enzyme replacement therapy for Hunter syndrome (MPS II).
elastin: A stretchy protein in connective tissue that allows skin, blood vessels, and lungs to snap back to their original shape.
electroporation: A brief electrical pulse applied after an injection that opens tiny pores in nearby cells so a DNA medicine can get inside them.
electroretinography: A test measuring the retina's electrical response to light flashes. Reduced responses indicate photoreceptor damage, helping diagnose RP.
elenestinib: An investigational KIT-blocking pill from Blueprint Medicines studied in systemic mastocytosis.
Elevidys: Brand name for delandistrogene moxeparvovec, a gene therapy for Duchenne muscular dystrophy that delivers a shortened form of the dystrophin gene.
eligibility criteria: The specific medical and demographic requirements for participating in a clinical trial.
ELISA: Enzyme-Linked Immunosorbent Assay, a common lab test that detects and measures specific proteins such as antibodies in blood or other fluids. It is the usual first-step screening test for Lyme disease.
elosulfase alfa: An enzyme replacement therapy (brand name Vimizim) that provides the missing N-acetylgalactosamine-6-sulfatase enzyme in patients with MPS IVA (Morquio A).
EMA: The European Medicines Agency, responsible for evaluating and approving medicines for use in the European Union.
emapalumab: A monoclonal antibody targeting interferon-gamma (brand name Gamifant). Approved for primary HLH and being studied for MAS in sJIA.
EMBARK: Sarepta's 125-boy placebo-controlled trial of Elevidys in boys aged 4 to 7, which missed its primary North Star endpoint (p = 0.24) but favored the drug on timed tests.
embolism: A blockage in a blood vessel caused by a clot, air bubble, or other material that has traveled from elsewhere in the body.
Emcitate: The brand name for tiratricol, Egetis Therapeutics' thyroid hormone analog for MCT8 deficiency. Approved in the EU in February 2025 and by the FDA on September 28, 2026, the first US treatment for the disease.
EMG: Electromyography — a test measuring the electrical activity in muscles.
emicizumab: The generic name of Hemlibra, a factor VIIIa mimetic antibody injected under the skin to prevent bleeds in hemophilia A, approved from birth with or without inhibitors.
emphysema: A lung disease where the tiny air sacs (alveoli) are damaged and enlarged, making it harder to breathe out and get enough oxygen.
EN-001: An investigational mesenchymal stem cell therapy from ENCell (Republic of Korea) for CMT1A. Phase 1 in 9 patients showed measurable improvement in CMT Neuropathy Score, walking tests, and nerve conduction at 16 weeks. Phase 1b/2a enrolling. FDA Orphan Drug Designation in March 2025.
encephalitis: Inflammation of the brain, usually caused by infection or an autoimmune response, leading to headache, confusion, and sometimes seizures.
encephalopathy: A broad term for any disease that affects brain function or structure, causing confusion, personality changes, or loss of consciousness.
endemic: A disease that occurs regularly in a specific geographic area or population.
endocarditis: Infection or inflammation of the inner lining of the heart chambers and valves.
endocrine: Relating to the system of glands that produce hormones which regulate metabolism, growth, reproduction, and other body functions.
endocrinologist: A doctor specializing in hormonal and metabolic disorders. The primary specialist for acromegaly and other pituitary conditions.
endogenous: Originating from within the body, as opposed to coming from an external source.
endometriosis: A condition where tissue like the uterus lining grows outside the uterus, causing pain and sometimes infertility.
endomyocardial biopsy: A heart biopsy in which tiny samples of heart muscle are taken through a catheter in a vein. In ATTR it is needed when blood tests or a bone scan cannot settle the diagnosis.
endoscopic: Performed using an endoscope, a thin flexible tube with a camera used to examine the inside of the body.
endoscopy: A procedure using a thin, flexible tube with a camera to examine the inside of the digestive system.
endothelial: Relating to the endothelium, the thin layer of cells lining the inside of blood vessels and the heart.
endothelin: A protein that tightens blood vessels and promotes inflammation and scarring. Blocking it can help protect the kidneys.
endothelin receptor antagonist: A drug that blocks endothelin, a protein that constricts blood vessels and promotes scarring. Used to reduce protein in the urine and protect the kidneys.
endothelium: The single layer of cells lining the inside of blood vessels and the heart, playing a key role in blood flow and vessel health.
endpoint: A measurable outcome in a clinical trial used to determine whether a treatment is working.
enteritis: Inflammation of the small intestine, often causing diarrhea, cramping, and malabsorption of nutrients.
entrectinib: A TRK and ROS1 inhibitor (brand name Rozlytrek) approved for NTRK fusion cancers. It can also reach tumors in the brain.
enzyme deficiency: A condition where the body does not produce enough of a specific enzyme needed for normal cell function.
enzyme replacement therapy: Treatment where missing or defective enzymes are supplied externally to restore normal cellular function.
eosinophilia: An abnormally high number of eosinophils (a type of white blood cell) in the blood, often associated with allergic conditions, parasitic infections, or eosinophilic diseases.
eosinophilic granulomatosis with polyangiitis: A rare vasculitis characterized by asthma, eosinophilia, and inflammation of small-to-medium blood vessels. Formerly called Churg-Strauss syndrome.
EPICS-III: Zydus's Phase 2b/3 trial of saroglitazar in primary biliary cholangitis. In its 148-patient Phase 3 part, 56.7% reached a lab response at 1 year against 9.8% on placebo.
epigenetic: Changes that affect how genes are turned on or off without altering the DNA sequence itself, influenced by environment and lifestyle.
epilepsy: A brain disorder characterized by recurrent seizures caused by abnormal electrical activity in the brain.
epinephrine: A hormone (also called adrenaline) released during stress that increases heart rate and blood pressure. Used as emergency treatment for severe allergic reactions.
eplontersen: A monthly self-injected antisense oligonucleotide (brand name Wainua) that lowers blood transthyretin by about 80%. Approved for hereditary ATTR polyneuropathy.
epsilon wave: A small extra signal seen on an ECG, characteristic of arrhythmogenic right ventricular cardiomyopathy (ARVC). Indicates delayed electrical activity in the heart.
equine: From horses.
eratrectinib: A next-generation TRK inhibitor for NTRK fusion cancers, approved in China in June 2026. It has not been filed for approval in the US.
ERG: Electroretinography, a test that measures how your retina responds to light. Used to diagnose and monitor retinitis pigmentosa.
ERISA: The federal law governing most employer health plans. Its claims rules set the appeal deadlines: 180 days to file, 30 days for a pre-service appeal decision, 72 hours if urgent.
erythema: Redness of the skin caused by increased blood flow, often a sign of inflammation or infection.
erythrocyte: Red blood cell — carries oxygen through your body.
erythromelalgia: A rare condition causing episodes of burning pain, warmth, and redness in the hands and feet. The inherited form is caused by mutations in sodium channel genes that make pain-sensing nerves overactive.
erythropoiesis: The process by which the body produces new red blood cells.
erythropoietin: A hormone the kidneys release when tissues are short of oxygen. It tells the bone marrow to make more red blood cells.
ESI: Emergency Severity Index; a 5-level triage system used in U.S. emergency departments. ESI 1 is most urgent (resuscitation), ESI 5 is least urgent.
esophageal: Relating to the esophagus, the tube that carries food from the throat to the stomach.
esophagitis: Inflammation of the esophagus, the tube connecting mouth to stomach. In eosinophilic esophagitis, allergy-type immune cells drive the inflammation.
ESSENCE: Sarepta's 225-boy confirmatory trial for Vyondys 53 and Amondys 45, which measured 4-step climb speed over 96 weeks and missed its primary endpoint in November 2025 (p = 0.309).
estrogen: A group of female sex hormones responsible for development of female characteristics and regulation of the reproductive cycle.
etavopivat: An investigational once-daily pill from Novo Nordisk that activates pyruvate kinase in red blood cells, tested in sickle cell disease in the HIBISCUS trial.
etiology: The cause or origin of a disease. Identifying the etiology helps guide treatment decisions.
ETV6-NTRK3: The most common NTRK fusion, joining the ETV6 gene to NTRK3. It defines secretory carcinoma and infantile fibrosarcoma and is the one NTRK3 fusion that DNA-only panels reliably catch.
evinacumab: An antibody (brand Evkeeza) that blocks ANGPTL3, dramatically lowering cholesterol in homozygous familial hypercholesterolemia.
Evrysdi: Brand name for risdiplam, a daily oral liquid treatment for spinal muscular atrophy in patients two months of age and older.
exagamglogene autotemcel: A CRISPR/Cas9 gene-edited cell therapy (brand name Casgevy) approved for sickle cell disease and beta thalassemia. The first approved CRISPR-based therapy.
exchangeable copper: A laboratory measurement of copper that is loosely bound in the blood and available for tissue uptake. Used in some specialty centers as a refined biomarker for Wilson disease management.
excipient: An inactive ingredient in a drug formulation, such as fillers, coatings, or preservatives, that help deliver the active ingredient.
exclusion criteria: Conditions or factors that prevent a patient from participating in a clinical trial.
exon: A segment of a gene that is kept in the final instructions for making a protein, after the stretches between exons are cut out. Genes vary widely, from 79 exons in dystrophin, the Duchenne gene, to 4 in TTR, the hereditary ATTR gene.
exon skipping: A therapeutic approach where certain portions of a gene are skipped during protein production to restore partial function.
expanded access: A pathway allowing patients to use investigational drugs before FDA approval for life-threatening conditions.
expressivity: How severely a genetic disease affects different people with the same mutation.
external review: An appeal decided by an independent review organization outside your insurer. Available after an internal appeal for denials based on medical judgment; the decision binds the plan and must come within 45 days, or 72 hours if expedited.
Extra Help: Medicare's Low-Income Subsidy, run through Social Security, which pays part of Part D premiums, deductibles and copays for people under set income and resource limits.
extracellular matrix: The structural network of proteins and sugars surrounding cells that provides support, like the scaffolding that holds tissue together.
extrahepatic: Located or occurring outside the liver.
extramedullary: Occurring outside the bone marrow. Extramedullary hematopoiesis means blood cells are being produced in abnormal locations.
extraocular: Located or occurring outside the eye, such as the extraocular muscles that control eye movement.
extravasation: Leakage of an IV drug out of the vein into surrounding tissue, which can cause pain and tissue damage.
extravascular: Outside the blood vessels. Extravascular fluid accumulation causes tissue swelling.

F

Fabhalta: Brand name for iptacopan, an oral treatment for paroxysmal nocturnal hemoglobinuria that works by inhibiting complement factor B.
FACETS trial: A Phase III clinical trial of migalastat (Galafold) in Fabry disease patients with amenable GLA mutations.
factor VIIIa mimetic: An antibody that does the job of activated factor VIII by holding factor IXa and factor X together, without being factor VIII. Hemlibra is one; denecimig is another. Because they are not factor VIII, inhibitors against factor VIII do not block them.
failure to thrive: A medical term for an infant or child who is not gaining weight or growing as expected.
fair hearing: A Medicaid appeal decided by a state hearing officer who does not work for your managed care plan. Federal rules require states to accept requests for up to 90 days after the denial notice.
fasciculations: Visible twitching or rippling under the skin caused by involuntary muscle contractions.
fast track: An FDA program that expedites the development and review of drugs addressing serious diseases and filling unmet medical needs.
Fayuvi: The brand name for rebisufligene etisparvovec, formerly UX111. A one-time AAV9 gene therapy approved in September 2026 as the first treatment for Sanfilippo syndrome type A, for children with preserved neurodevelopmental function.
FcRn: Neonatal Fc Receptor — a protein that recycles antibodies (IgG) in the body. Blocking FcRn causes harmful autoantibodies to break down faster.
FDA-approved: Officially recognized and permitted by the U.S. Food and Drug Administration for use in treating a specific condition.
felzartamab: An investigational antibody that targets CD38, a protein on the antibody-producing plasma cells; in IgA nephropathy it is meant to cut production of the abnormal IgA at its source. In Phase 3 trials as of 2026.
fenebrutinib: An investigational brain-penetrant BTK inhibitor from Roche with positive Phase 3 results in relapsing and primary progressive multiple sclerosis.
ferritin: A blood protein that stores iron. The ferritin level is used to gauge iron stores and, when sharply elevated, as a marker of inflammation.
fetal hemoglobin: The form of hemoglobin made before birth. It does not sickle, so treatments that raise it can protect red cells in sickle cell disease.
FGFR inhibitor: A drug that blocks FGFR growth-signal receptors. Most block several FGFR types at once, which causes side effects like high blood phosphate.
FGFR1: A related growth-signal receptor (fibroblast growth factor receptor 1) that helps the body regulate phosphate. Drugs that block it tend to raise blood phosphate levels.
FGFR2: A gene that makes a protein (fibroblast growth factor receptor 2) telling cells when to grow. When FGFR2 abnormally fuses with another gene, it can act like a stuck accelerator and drive cancers such as cholangiocarcinoma.
FGFR3: A growth-receptor gene that acts as a brake on bone growth. Overactive FGFR3 causes achondroplasia, the most common form of dwarfism.
FGFR4: A related growth-signal receptor (fibroblast growth factor receptor 4) active in the liver and gut. Blocking it is linked to side effects such as diarrhea.
fibrillin-1: A protein that provides structural support in connective tissue. Mutated in Marfan syndrome.
fibrin: A fibrous protein that forms a mesh during blood clotting, creating the structural framework of a blood clot.
fibro-fatty: Replacement of normal tissue with a mix of fibrous (scar) tissue and fat. In heart disease, this weakens the heart muscle wall.
fibroblast: Cells in connective tissue that produce collagen and other structural proteins. Overactive fibroblasts can cause excessive scarring (fibrosis).
fibrodysplasia ossificans progressiva: An ultra-rare genetic disease in which muscle and connective tissue gradually turn into bone, progressively locking joints in place. Also called FOP.
fibrosis: Scarring and thickening of tissue, often as a result of inflammation or injury.
fidanacogene elaparvovec: A gene therapy (brand name BEQVEZ) that delivers a working copy of the Factor IX gene to treat hemophilia B with a single IV infusion.
fine-needle aspiration: A biopsy technique using a very thin needle to withdraw a small sample of cells from a lump or mass for examination.
FIPA: Familial isolated pituitary adenoma: pituitary tumors that run in a family without the other hormone tumors seen in syndromes like MEN1. About 10% to 15% of such families carry an AIP variant.
first-line: The first treatment given for a disease. Usually the standard, most proven option.
FISH: Fluorescence In Situ Hybridization, a lab test that uses fluorescent probes to detect specific gene changes like fusions or deletions in cells.
fitusiran: The generic name of Qfitlia, a small interfering RNA injected under the skin as seldom as every 2 months that lowers antithrombin to prevent bleeds in hemophilia A or B from age 12.
FKRP: Fukutin-related protein, an enzyme that helps attach sugar chains to alpha-dystroglycan in muscle. Mutations in its gene cause limb-girdle muscular dystrophy type 2I/R9.
flare: A period when disease symptoms suddenly get worse after being stable or in remission.
flow cytometry: A lab technique that rapidly analyzes thousands of individual cells, identifying their type and characteristics by how they scatter laser light.
fluoroquinolone: A class of antibiotics (ciprofloxacin, levofloxacin) that carry FDA warnings for neuromuscular side effects and can worsen myasthenia gravis.
fluoroscopy: A type of continuous X-ray imaging that shows real-time moving images of internal structures, like a live X-ray video.
FMTC: Familial Medullary Thyroid Cancer — an inherited form of MTC caused by RET gene mutations, where thyroid cancer is the only feature (unlike MEN 2 syndromes).
focal: Limited to one specific area. A focal seizure, for example, starts in one part of the brain rather than the whole brain.
follicular: Relating to follicles, small sac-like structures in the body such as hair follicles or thyroid follicles.
foot drop: Inability to lift the front part of the foot while walking, causing stumbling.
forced vital capacity: A lung function test measuring the total amount of air you can forcefully exhale after a full breath. It tracks scarring lung diseases such as pulmonary fibrosis and breathing-muscle weakness in conditions like Pompe disease.
formulary: An insurer's list of covered drugs, organized into tiers that determine your copay and whether extra approval steps apply.
formulary exception: A request, backed by a statement from your prescriber, asking a drug plan to cover a drug that isn't on its list or to waive a rule like step therapy. Medicare plans must answer within 72 hours, or 24 if urgent.
formulation: The specific physical form of a medication (tablet, liquid, injection, patch, etc.) and the inactive ingredients it contains.
FORTIFY: BridgeBio's Phase 3 trial of BBP-418 in 112 people aged 12 to 60 with limb-girdle muscular dystrophy type 2I/R9. Its 12-month interim analysis supports the FDA application; the final motor result comes at 36 months.
FORZETTO: Dyne's Phase 3 confirmatory trial of zeleciment rostudirsen, recruiting about 90 boys aged 4 to 18 and comparing rise-from-floor speed at week 73 against placebo.
Forzinity: Brand name of elamipretide, approved by the FDA in 2025 to improve muscle strength in Barth syndrome. It is approved only for Barth syndrome, a mitochondrial disorder, and not for Leigh syndrome.
FOXG1: A master gene of early brain development. FOXG1 syndrome, related to Rett syndrome, causes severe developmental impairment from infancy.
frameshift mutation: A change in DNA where nucleotides are inserted or deleted, shifting the reading frame and disrupting protein production.
free copper: Copper in the blood not bound to ceruloplasmin. More chemically reactive and a closer reflection of tissue copper toxicity than total serum copper in Wilson disease.
free radical: Unstable molecules that damage cells through oxidative stress. The body produces them naturally, but excess free radicals can injure neurons and other tissues.
fSARA: Functional SARA, a 16-point version of the ataxia rating scale covering only gait, sitting, stance and speech, used by the FDA as the primary measure in the Aqneursa trials. Lower is better.
fulminant: Occurring suddenly and with great severity. A fulminant disease progresses rapidly and can be life-threatening.
fundus flavimaculatus: An alternative name for the later-onset form of Stargardt disease, characterized by yellowish flecks in the retina visible on examination.
futibatinib: An FGFR-blocking pill (brand Lytgobi) approved in 2022 for bile duct cancer inside the liver with an FGFR2 fusion. It binds its target permanently.
FVC: Forced vital capacity, a lung function test measuring how much air you can forcefully breathe out. IPF drug trials use its yearly decline, in mL, as the main measure of benefit.

G

G6PD: Glucose-6-phosphate dehydrogenase; an enzyme that protects red blood cells from damage. Deficiency causes risk of hemolytic anemia with certain drugs like tafenoquine.
GAA gene: The gene responsible for producing acid alpha-glucosidase enzyme. Mutations in this gene cause Pompe disease.
GAG: Glycosaminoglycans, complex sugar molecules that accumulate in MPS diseases due to enzyme deficiency, damaging tissues and organs.
GAG accumulation: The buildup of glycosaminoglycan molecules in tissues throughout the body, the underlying cause of damage in MPS diseases.
gain of function: A mutation that gives a protein an abnormal ability or excessive activity, causing disease.
gait abnormality: Irregular or abnormal pattern of walking.
galactose-deficient IgA1: An abnormal form of the IgA1 antibody missing normal galactose sugar molecules. The immune system produces autoantibodies against it, forming complexes that deposit in the kidneys and cause IgA nephropathy.
galactosemia: A genetic disorder where the body cannot properly process galactose (a sugar in milk), causing toxic buildup that damages organs.
Galafold: Brand name for migalastat, an oral treatment for Fabry disease that helps the body's own enzyme work more effectively.
gallbladder: A small organ under the liver that stores bile. Somatostatin analog medications can increase the risk of gallstones by slowing gallbladder emptying.
gallstone: Hard deposits that form in the gallbladder. A known side effect of somatostatin analog medications used in acromegaly.
GalNAc: N-acetylgalactosamine, a sugar attached to some RNA drugs so that liver cells pull them in. It is why vutrisiran and eplontersen work as small injections under the skin.
galsulfase: An enzyme replacement therapy (brand name Naglazyme) that provides the missing arylsulfatase B enzyme in patients with MPS VI.
gammopathy: A condition where the body produces abnormal immunoglobulin (antibody) proteins, which can indicate blood cancers or be benign.
gangliosidosis: A group of inherited diseases where fatty substances called gangliosides accumulate in nerve cells, causing progressive neurological damage.
garadacimab: An antibody (brand Andembry) that blocks factor XIIa, given monthly to prevent hereditary angioedema attacks.
GARD: NIH Genetic and Rare Diseases Information Center — provides evidence-based information on rare diseases.
garetosmab: An antibody (brand Pasatru) that blocks activin A, approved in August 2026 for adults with FOP. It was the first FOP treatment proven in a placebo-controlled trial; Sohonos (palovarotene) was the first FOP drug approved, in 2023.
gastrectomy: Surgical removal of part or all of the stomach, sometimes performed for cancer or severe ulcer disease.
gastric: Relating to the stomach.
gastritis: Inflammation of the stomach lining, which can cause pain, nausea, and sometimes bleeding.
gastrointestinal: Relating to the stomach and intestines (digestive system).
gastroparesis: Delayed stomach emptying due to nerve damage, causing nausea, vomiting, and bloating.
Gb3: Globotriaosylceramide — a fatty substance that accumulates in the cells of Fabry disease patients, causing progressive organ damage.
Gd-IgA1: Galactose-deficient IgA1, an abnormally formed antibody. The immune system treats it as foreign and attacks it, and the resulting clumps deposit in the kidneys and cause the inflammation behind IgA nephropathy.
gefurulimab: A C5-blocking nanobody from AstraZeneca, sold as Klygefa in Japan and given as a once-weekly self-injection with an autoinjector. It is under FDA review for adults with AChR antibody-positive generalized myasthenia gravis.
gelatine: A protein made from cattle and pig collagen, used in capsule shells, softgels, and as a vaccine stabilizer. The most common source of alpha-gal in medications.
gene expression: The process by which information in a gene is used to produce a functional protein. Changes in expression can drive disease.
gene fusion: An abnormal joining of two genes, creating a hybrid that can produce a growth-driving protein. Many targeted cancer drugs work by blocking the protein made by a specific gene fusion.
gene panel: A genetic test that screens many disease-related genes at once using a single blood sample. CMT panels typically cover 50 or more CMT-associated genes.
gene silencing: The process of blocking a gene from being expressed or making proteins.
gene therapy: Treatment that works by introducing genetic material into cells to correct disease-causing mutations.
generalized myasthenia gravis: Myasthenia gravis that weakens muscles beyond the eyes, such as those used to swallow, speak, breathe and move the arms and legs. The targeted MG drugs are approved for this form.
GeneReview: Expert-authored, peer-reviewed articles about genetic conditions for healthcare providers and patients.
genetic counseling: Professional guidance to help families understand genetic risks and inheritance patterns.
genetic disorder: A disease caused by abnormal genes or genetic mutations inherited from parents or acquired during life.
genetic testing: Laboratory analysis of DNA to identify mutations or variants in specific genes that may cause or contribute to a disease.
genetic variant: A change in a gene's DNA sequence that may or may not affect how the gene works.
geneticist: A doctor specializing in genetic conditions and hereditary diseases. Often coordinates care for patients with rare genetic disorders like Fabry or Pompe disease.
genotype: Your specific genetic makeup for a particular gene.
germline: Present in the DNA a person is born with, carried in every cell and passable to children, as opposed to a change that arose later in one tissue.
gigantism: Growth hormone excess that starts in childhood, before the growth plates in the bones close, so the person grows unusually tall. The same excess in adulthood causes acromegaly.
givinostat: An oral HDAC inhibitor (brand Duvyzat) approved for Duchenne muscular dystrophy in patients 6 and older, regardless of their dystrophin mutation.
GJB1: A gene on the X chromosome that codes for connexin 32, a protein important to myelin function. Mutations cause CMTX1 (the X-linked form of CMT).
GL-3: Globotriaosylceramide — a fatty substance that builds up in cells when the alpha-galactosidase A enzyme is missing or deficient, as in Fabry disease.
GLA gene: The gene that provides instructions for making the alpha-galactosidase A enzyme. Mutations in this gene cause Fabry disease.
GLA mutation: A change in the GLA gene that causes the alpha-galactosidase A enzyme to be absent or dysfunctional, leading to Fabry disease.
Glassia: Takeda's liquid alpha-1 proteinase inhibitor for AATD, licensed from Kamada and approved in 2010; its label allows trained self-infusion at home.
glaucoma: A group of eye diseases that damage the optic nerve, often due to abnormally high pressure inside the eye, leading to vision loss.
gliosis: Scarring in the brain or spinal cord caused by the buildup of glial cells in response to damage or disease.
glomerular: Relating to the glomeruli, the filtering units in the kidneys.
glomerulonephritis: Inflammation of the tiny filters in the kidneys, which can impair kidney function.
glomerulopathy: Disease of the glomeruli, the kidney's microscopic filtering units.
glomerulosclerosis: Scarring and hardening of the tiny blood-filtering units (glomeruli) in the kidneys, leading to progressive kidney failure.
glucagon: A hormone produced by the pancreas that raises blood sugar levels, acting as the opposite of insulin.
glucocerebrosidase: An enzyme that breaks down a fatty chemical in cells. Deficient in Gaucher disease.
glucocerebroside: A fatty substance that accumulates in cells when the enzyme to break it down is missing, as in Gaucher disease.
glucocorticoid: A type of corticosteroid hormone or drug that reduces inflammation and immune activity. Prednisone and deflazacort are common examples.
glucosuria: Glucose (sugar) in the urine. In Wilson disease, this reflects kidney tubule dysfunction rather than diabetes.
glutamate: The brain's main excitatory chemical messenger. Too much glutamate can overexcite and kill nerve cells, a process called excitotoxicity.
glycogen: A complex carbohydrate stored in muscles and the liver for energy.
glycosaminoglycan: Complex sugar molecules (GAGs) that build up in MPS diseases when the enzyme to break them down is missing, causing progressive organ damage.
GM-CSF: Granulocyte-macrophage colony-stimulating factor, a signal protein that tells the lungs' cleanup cells to clear used surfactant and fight germs. Antibodies against it cause autoimmune pulmonary alveolar proteinosis, and inhaled molgramostim is a manufactured replacement.
GM1: A fatty molecule (ganglioside) that builds up in nerve cells when its recycling enzyme is missing, causing GM1 gangliosidosis.
GM2: A fatty molecule (ganglioside) that builds up in nerve cells in Tay-Sachs and Sandhoff disease, together called the GM2 gangliosidoses.
GMFM-88: Gross Motor Function Measure, an 88-item test of movement skills such as rolling, sitting, crawling and walking, scored by a trained observer. Used as a primary endpoint in Triac Trial II.
govorestat: An aldose reductase inhibitor (development code AT-007) tested in the Phase 2/3 INSPIRE trial for CMT-SORD. Developed by Applied Therapeutics, which Cycle Pharmaceuticals acquired in 2026.
Gowers sign: A characteristic movement where a child with muscular dystrophy has to use their hands to climb up their legs to stand.
grade 3-5: A serious or life-threatening side effect on the standard grading scale.
grading: A system for classifying how abnormal cancer cells look under a microscope, which helps predict how fast the cancer will grow.
graft-versus-host: A complication after transplant where donated immune cells attack the recipient's body, seeing it as foreign.
graft-versus-host disease: A condition where transplanted donor immune cells attack the recipient's healthy tissues. Can be acute (early) or chronic (late-onset), affecting skin, eyes, mouth, lungs, liver, and GI tract.
gram-negative: A category of bacteria with a thin cell wall that doesn't retain Gram stain dye. Includes many common pathogens. Often harder to treat because the outer membrane blocks some antibiotics.
granuloma: A small cluster of immune cells that forms when the body tries to wall off substances it sees as foreign, found in several diseases.
granulomatosis: A condition involving the formation of granulomas (small clusters of immune cells) that can damage blood vessels and organs.
growth hormone: A hormone produced by the pituitary gland that regulates body growth and metabolism. Overproduction in adults causes acromegaly.
growth plate: Strips of cartilage near the ends of a child's long bones where new bone forms. They usually close in the teenage years, after which bones stop growing in length and growth treatments stop working.

H

half-life: The time it takes for half of a drug to be eliminated from your body.
hammer-toe: Toes bent into a permanent claw or hammer shape, caused by long-standing muscle imbalance in the foot. A characteristic feature of CMT and similar neuromuscular conditions.
Hammersmith Functional Motor Scale-Expanded: A 66-point scale used in spinal muscular atrophy where a clinician scores 33 physical tasks, such as rolling over, sitting unsupported, standing and climbing a step. Usually shortened to HFMSE.
haploinsufficiency: A condition where one functional copy of a gene is not enough to maintain normal function.
hATTR: Hereditary transthyretin amyloidosis: the inherited form, caused by a variant in the TTR gene, which can affect the nerves, the heart or both.
hazard ratio: A number comparing how quickly events (like disease progression) happen in the treatment group versus the control group. Below 1.0 favors treatment.
HbA1c: A blood test showing your average blood sugar over the past 2-3 months.
heart failure: A chronic condition where the heart cannot pump blood efficiently enough to meet the body's needs, causing fatigue and fluid buildup.
hemarthrosis: Bleeding into a joint space, causing pain and swelling. Common in hemophilia and other bleeding disorders.
hematological: Relating to the blood and blood-forming organs.
hematopoiesis: The process by which the body produces new blood cells (red cells, white cells, and platelets) in the bone marrow.
hematopoietic: Relating to the formation of blood cells. Hematopoietic stem cells in bone marrow produce all types of blood cells.
hematuria: Blood in the urine, which can indicate kidney or urinary tract disease.
hemochromatosis: A condition where the body absorbs too much iron from food, causing iron to build up and damage the liver, heart, and other organs.
hemodynamic: Relating to blood flow and the forces involved in circulating blood through the body.
hemoglobin: A protein in red blood cells that carries oxygen. Low levels = anemia.
hemoglobin response: A trial goal counting the people whose hemoglobin rose by a set amount, usually at least 1 g/dL, over a set period.
hemoglobinopathy: A group of inherited disorders affecting the structure or production of hemoglobin in red blood cells.
hemoglobinuria: Hemoglobin in the urine, often turning it dark, a sign that red blood cells are breaking apart in the bloodstream.
hemolysis: The destruction or breakdown of red blood cells, which releases hemoglobin into the bloodstream.
hemolytic anemia: A condition where red blood cells are destroyed faster than they can be made, causing fatigue and shortness of breath.
hemophagocytic lymphohistiocytosis: A rare, life-threatening condition in which the immune system becomes massively overactivated and attacks the body's own organs. Also called HLH.
hemoptysis: Coughing up blood or bloody mucus from the lungs or airways.
hemorrhage: Significant bleeding, either inside the body (internal) or outside (external).
heparan sulfate: A type of glycosaminoglycan (complex sugar chain) that accumulates in MPS diseases when the enzyme to break it down is missing.
heparin: A blood-thinning medication that prevents clots from forming, commonly used during surgery and to treat blood clots.
hepatic: Relating to the liver.
hepatitis: Inflammation of the liver, which can be caused by viruses, alcohol, drugs, or autoimmune conditions.
hepatitis B reactivation: A dormant hepatitis B infection flaring back up after a drug lowers immune defenses. B-cell depleting drugs can trigger it, so all of them require a hepatitis B blood test before the first dose.
hepatobiliary: Relating to the liver and bile ducts, the system that produces, stores, and transports bile for digestion.
hepatocellular: Relating to the main cells of the liver (hepatocytes). Hepatocellular carcinoma is the most common type of liver cancer.
hepatocellular carcinoma: The most common type of primary liver cancer, arising from the liver's main working cells (hepatocytes).
hepatocellular injury: Damage to the liver's main working cells, usually found as rising liver enzymes on blood tests.
hepatocyte: The main functional cells of the liver, responsible for processing nutrients, filtering toxins, and producing bile.
hepatomegaly: Enlargement of the liver beyond normal size.
hepatosplenomegaly: Enlargement of both the liver and spleen, often caused by abnormal material accumulation in storage disorders.
hepatotoxic: Capable of causing damage to the liver. Liver function tests are monitored for drugs known to be hepatotoxic.
hepatotoxicity: Liver damage caused by drugs, chemicals, or other substances. A common side effect monitored in clinical trials.
HER2: A growth-signal receptor that some tumors make in overdrive. HER2-targeted drugs block that signal.
hereditary: Passed down from parent to child through genes. Hereditary cancers run in families due to inherited gene mutations.
Herxheimer: Short for Jarisch-Herxheimer reaction; a temporary inflammatory flare during treatment as pathogens die off.
heteroplasmy: The mix of normal and mutated mitochondrial DNA copies inside a cell. In mitochondrial DNA diseases, a higher share of mutated copies usually means more severe symptoms.
heterotopic ossification: Bone that forms where bone should not be, such as inside muscle or tendon. The hallmark of FOP, usually measured on CT scans as new 'lesions.'
heterozygous: Carrying 1 copy of a particular gene variant, from 1 parent, alongside a typical copy from the other parent.
HFMSE: Hammersmith Functional Motor Scale-Expanded. A 66-point scale used in spinal muscular atrophy where a clinician scores 33 physical tasks, such as rolling over, sitting unsupported, standing and climbing a step.
HIBISCUS: Novo Nordisk's Phase 3 trial of etavopivat in 385 people aged 12 and older with sickle cell disease.
high-dose chemotherapy: Strong chemotherapy that clears the bone marrow so new or modified stem cells can take its place. Also called myeloablative conditioning.
hip dysplasia: An abnormal shape or development of the hip joint, increasing risk of pain and dislocation. Can occur in childhood-onset CMT and other neuromuscular conditions.
hippocampus: A brain structure essential for memory formation and spatial navigation.
histamine: A chemical released by immune cells during allergic reactions, causing itching, swelling, redness, and other allergy symptoms.
histologically confirmed: Confirmed by examining a tissue sample under a microscope.
histopathology: The study of diseased tissue under a microscope, used to diagnose diseases and determine how severely tissue is affected.
historical control: Patients from an earlier study or registry whose outcomes are used as the comparison for a new treatment, instead of a placebo group enrolled at the same time. Weaker evidence than a randomized comparison because the groups may differ in ways nobody measured.
Holter monitor: A small wearable recorder that tracks every heartbeat for a day or more to catch rhythm problems a short ECG would miss.
homocysteine: An amino acid in the blood. High levels can damage blood vessels and increase risk of heart disease and stroke.
homozygous: Having 2 identical copies of a gene variant (1 from each parent).
HOPE-3: Capricor's Phase 3 trial of deramiocel in 106 boys and young men with Duchenne, most no longer able to walk. Capricor's analysis showed slower loss of upper limb function; the FDA's analysis of the original plan did not reach significance.
HPO: Human Phenotype Ontology — a standardized database of clinical features and symptoms mapped to rare diseases.
HSCT: Hematopoietic Stem Cell Transplant — a procedure using stem cells to restore blood cell production.
Hurler syndrome: The most severe form of MPS I with early onset (by age 2-3), rapid progression, and significant cognitive decline without treatment.
Hurler-Scheie: An intermediate form of MPS I with childhood onset and variable progression between the severe (Hurler) and mild (Scheie) forms.
hyaluronidase: An enzyme that briefly breaks down hyaluronic acid, the gel that fills the space under the skin, so an injected drug or fluid spreads and absorbs faster. Some versions are purified from cow or sheep testes; others are human hyaluronidase made in cell culture.
hydrocephalus: Buildup of fluid inside the brain's ventricles, increasing pressure on the brain and potentially causing damage.
hydrocortisone: A synthetic form of cortisol used as replacement therapy in adrenal insufficiency and as emergency treatment during adrenal crisis.
hyperaldosteronism: Overproduction of aldosterone by the adrenal glands, causing high blood pressure and low potassium levels.
hyperammonemia: Dangerously elevated ammonia levels in the blood, which is toxic to the brain and can cause confusion, seizures, or coma.
hyperbilirubinemia: Elevated bilirubin levels in the blood, which can cause yellowing of the skin and eyes (jaundice).
hypercalcemia: Abnormally high calcium levels in the blood, which can weaken bones, cause kidney stones, and affect heart rhythm.
hypercalciuria: Abnormally high levels of calcium in the urine, which can lead to kidney stones.
hypercapnia: An abnormally high level of carbon dioxide in the blood, usually from the lungs not ventilating well enough.
hypercholesterolemia: Abnormally high cholesterol levels in the blood, increasing the risk of heart disease and stroke.
hypercortisolism: Excess cortisol production (also called Cushing's syndrome), causing weight gain, high blood pressure, and metabolic problems.
hyperglycemia: Abnormally high blood sugar levels, a hallmark of diabetes that can damage blood vessels and organs over time.
hyperkalemia: Dangerously high potassium levels in the blood, which can cause life-threatening heart rhythm abnormalities.
hypermobility: Joints that bend further than normal, often caused by unusually flexible connective tissue. Can lead to joint pain and dislocations.
hyperopia: Farsightedness — close objects appear blurry while distant objects are clearer.
hyperparathyroidism: Overactivity of the parathyroid glands causing high calcium levels. Can occur as part of MEN 2A syndrome.
hyperphagia: An overwhelming, insatiable appetite and drive to eat, often seen in Prader-Willi syndrome and certain brain injuries.
hyperphosphatemia: Too much phosphate in the blood. A common side effect of drugs that block FGFR1, managed with diet, phosphate binders or dose changes.
hyperpigmentation: Darkening of the skin caused by excess melanin production, appearing as patches darker than the surrounding skin.
hyperplasia: An increase in the number of cells in a tissue or organ, causing it to grow larger.
hyperreflexia: Abnormally exaggerated reflex responses.
hypersensitivity: An allergic or exaggerated immune reaction to a substance.
hypertension: High blood pressure, a condition where the force of blood against artery walls is consistently too high.
hyperthyroidism: An overactive thyroid gland that produces too much thyroid hormone, causing weight loss, rapid heartbeat, and anxiety.
hypertonia: Abnormally increased muscle tone or stiffness, making movement difficult.
hypertrichosis: Excessive hair growth in areas where hair is not normally prominent, which can be a sign of certain genetic or hormonal conditions.
hypertriglyceridemia: Abnormally high triglyceride (blood fat) levels, which increase the risk of heart disease and pancreatitis.
hypertrophy: Abnormal enlargement of a tissue or organ, often caused by the cells increasing in size.
hyperuricemia: Abnormally high uric acid levels in the blood, which can lead to gout and kidney stones.
hyperventilation: Breathing faster or deeper than normal, lowering carbon dioxide levels in the blood and potentially causing dizziness and tingling.
hyperviscosity: Abnormally thick blood that flows sluggishly, which can block small blood vessels and reduce oxygen delivery to tissues.
hypoalbuminemia: Abnormally low levels of albumin (a key blood protein), which can cause tissue swelling and indicate liver or kidney disease.
hypocalcemia: Abnormally low calcium levels in the blood, which can cause muscle cramps, numbness, and heart rhythm problems.
hypocretin: A brain chemical (also called orexin) that regulates wakefulness. Loss of hypocretin-producing neurons causes narcolepsy.
hypogammaglobulinemia: Abnormally low levels of antibodies (immunoglobulins) in the blood, increasing vulnerability to infections.
hypoglycemia: Abnormally low blood sugar levels, which can cause shakiness, confusion, and in severe cases, loss of consciousness.
hypogonadism: A condition where the body produces insufficient sex hormones, affecting puberty, fertility, and sexual function.
hypokalemia: Abnormally low potassium levels in the blood, which can cause muscle weakness, cramps, and heart rhythm problems.
hyponatremia: Abnormally low sodium levels in the blood, which can cause confusion, seizures, and in severe cases, brain swelling.
hypoparathyroidism: A rare condition where the parathyroid glands make too little parathyroid hormone, leaving blood calcium low and phosphorus high. It causes muscle cramps, numbness and tingling.
hypophosphatemia: Low phosphate levels in the blood, which weaken bones and muscles. X-linked hypophosphatemia is a rare genetic cause.
hypopigmentation: Lightening of the skin caused by reduced melanin production, appearing as patches lighter than the surrounding skin.
hypoplasia: Underdevelopment or lack of normal growth of a tissue or organ.
hypoplastic: Underdeveloped or abnormally small. Hypoplastic organs or tissues have not grown to their full size.
hyporeflexia: Abnormally diminished or absent reflex responses.
hypotension: Low blood pressure, which can cause dizziness, fainting, and in severe cases, organ damage from insufficient blood flow.
hypothalamus: A small brain region that controls hormone production, body temperature, hunger, thirst, and sleep-wake cycles.
hypothyroidism: An underactive thyroid gland that does not produce enough thyroid hormone, causing fatigue, weight gain, and cold sensitivity.
hypotonia: Abnormally low muscle tone, making muscles feel floppy or weak.
hypoventilation: Breathing that is too slow or too shallow to adequately exchange oxygen and carbon dioxide.
hypoxemia: An abnormally low level of oxygen in the blood, often measured by pulse oximetry or an arterial blood gas test.
hypoxia: A condition where body tissues do not receive enough oxygen, potentially causing organ damage.
hysterectomy: Surgical removal of the uterus.

I

ianalumab: An anti-BAFF receptor antibody being investigated for Sjögren's syndrome. BAFF is a protein that promotes B-cell survival and is elevated in autoimmune diseases.
iatrogenic: Caused by medical treatment or procedures. An iatrogenic condition is an unintended consequence of healthcare.
ibrutinib: A BTK inhibitor (brand name Imbruvica) used in chronic lymphocytic leukemia, mantle cell lymphoma, and chronic graft-versus-host disease. Works by blocking Bruton's tyrosine kinase.
ICD-10: The diagnosis coding system insurers use. An appeal should name the exact ICD-10 code for your condition, because a wrong or missing code is one of the commonest reasons a claim is denied.
ICER: The Institute for Clinical and Economic Review, an independent US nonprofit that estimates what a drug's health benefit is worth and publishes a fair-price range to compare with the list price.
ichthyosis: A group of inherited skin disorders in which the skin becomes dry, thick, and scaly because the outer layer does not shed normally.
ICV: Intracerebroventricular, meaning delivered directly into the brain's fluid spaces via a surgically placed catheter.
IDH1: A gene for a metabolic enzyme (isocitrate dehydrogenase 1). Certain IDH1 mutations drive some cancers, including a subset of bile duct cancers, and can be treated with IDH1-blocking drugs such as ivosidenib.
idiopathic: Of unknown cause — doctors haven't identified why it happened.
idiopathic pulmonary fibrosis: A lung disease of unknown cause in which scar tissue builds up and steadily reduces how much air the lungs can hold. It mostly affects adults over 60.
idursulfase: An enzyme replacement therapy (brand name Elaprase) for Hunter syndrome that supplies the missing iduronate-2-sulfatase enzyme.
IgA nephropathy: A kidney disease where IgA antibodies build up in the kidneys, causing inflammation and gradual damage to the filtering units. The most common form of glomerulonephritis worldwide.
IgA1: A type of antibody normally found in blood. In IgA nephropathy, an abnormal form of IgA1 deposits in the kidneys and triggers inflammation.
IGF-1: Insulin-like Growth Factor 1, a hormone stimulated by growth hormone. Elevated IGF-1 is the primary blood test for diagnosing and monitoring acromegaly.
IGF-1R: Insulin-like growth factor-1 receptor. A switch on the surface of cells that, when overstimulated, makes tissue behind the eye grow and swell. Blocking it is how Tepezza treats thyroid eye disease.
IgG: Immunoglobulin G; the most common antibody in blood. Develops later in infection and indicates past or ongoing exposure.
IgG degrader: A drug designed to break down and remove immunoglobulin G (IgG) antibodies from the blood. Used in autoimmune diseases where the patient's own IgG antibodies are attacking healthy tissue.
IgG4-RD: IgG4-Related Disease — a fibroinflammatory condition that can affect virtually any organ, characterized by elevated serum IgG4 levels and storiform fibrosis on biopsy.
IgG4-related disease: A chronic inflammatory condition where IgG4-producing immune cells infiltrate and scar multiple organs, most commonly the pancreas, salivary glands, and bile ducts.
IGHMBP2: A gene important to motor neuron and peripheral nerve function. Recessive mutations cause CMT2S, with AAV9-based gene therapy in early trials.
IgM: Immunoglobulin M; the first antibody produced during early infection. Indicates a recent or active immune response.
ignaseclant: An investigational oral ClC-1 (chloride ion channel) inhibitor (formerly NMD670) developed by NMD Pharma to boost muscle response in CMT Type 1 and Type 2. The Phase 2a SYNAPSE-CMT trial missed its primary endpoint (6-minute walk test) in February 2026.
IHC: Immunohistochemistry, a lab test that highlights specific proteins in tissue samples using tagged antibodies.
IL-36: Interleukin-36, a signaling pathway that switches on inflammation in the skin. It runs out of control in generalized pustular psoriasis, and Spevigo and imsidolimab both work by blocking its receptor.
ileum: The third and final section of the small intestine, where nutrients like vitamin B12 and bile salts are absorbed.
ileus: A temporary halt in the normal movement of the intestines, preventing food from passing through.
imaging tracer: A radioactive or contrast substance injected into the body that highlights specific tissues or tumors on a scan.
imatinib: A pioneering targeted cancer pill (brand Gleevec) that blocks the BCR-ABL protein in chronic myeloid leukemia and KIT in other conditions.
immune complex: Clusters formed when antibodies bind to their targets in the blood. These can deposit in tissues like the kidneys and trigger inflammation.
immune tolerance: The immune system's ability to recognize and not attack the body's own cells. Loss of tolerance leads to autoimmune disease.
immunoassay: A lab test that uses antibodies to detect or measure specific proteins, antigens, or antibodies in a sample.
immunocompromised: Having a weakened immune system that can't fight infections as well.
immunodeficiency: A weakened immune system that is less able to fight infections, either inherited (primary) or acquired (secondary).
immunofixation: A blood and urine test that looks for an abnormal antibody made by a single group of plasma cells. With the free light chain test, it is how doctors rule out AL amyloidosis.
immunofluorescence: A lab technique that uses glowing antibody tags to identify specific proteins in a tissue sample under a microscope. Used to diagnose autoimmune and skin diseases.
immunogenicity: The tendency of a therapeutic drug to trigger the body's immune system to produce antibodies against it, which may reduce the drug's effectiveness.
immunoglobulin: A protein (antibody) made by the immune system to fight infections. Different types (IgA, IgG, IgM, IgE) serve different protective roles.
immunohistochemistry: A lab technique that uses antibodies to detect specific proteins in tissue samples under a microscope, helping diagnose and classify diseases.
immunosuppressant: A drug that dampens the immune system, used to prevent organ rejection after transplants or treat autoimmune diseases.
immunosuppressed: Immune system is weakened, often by medications.
immunosuppression: Medications or therapies that reduce immune system activity.
immunotherapy: Treatment that helps your own immune system recognize and fight disease.
IMPALA-2: Savara's Phase 3 trial of inhaled molgramostim in 164 adults with autoimmune pulmonary alveolar proteinosis. The drug improved DLCO, a breathing test of oxygen transfer, by 9.8 points against 3.8 on placebo at week 24.
implantable: A medical device surgically placed inside the body for long-term treatment, such as a pacemaker or drug delivery port.
important potential risk: A regulatory label for a side effect that may be caused by a drug but has not been confirmed. It triggers extra monitoring in trials.
imsidolimab: An investigational antibody that blocks the IL-36 receptor, developed for generalized pustular psoriasis flares.
IMVT-1402: An investigational FcRn-blocking antibody from Immunovant, given as an injection under the skin, in Phase 3 testing for generalized myasthenia gravis.
in situ: In its original place; has not spread to surrounding tissue.
in vitro: Testing done in a lab (in test tubes or dishes) rather than inside a living body. Results may not directly translate to human treatment.
inactive ingredient: Everything in a pill besides the drug itself: the capsule shell, fillers, coatings, and colorings. Chosen by each manufacturer, which is why the same drug differs between makers.
incidence: The number of new cases of a disease diagnosed within a specific time period, indicating how quickly a disease is spreading.
inclusion criteria: The list of requirements a patient must meet to be eligible to join a clinical trial.
IND: Investigational New Drug application — the FDA submission allowing a company to begin human testing of a new drug.
indication: A medical condition or disease for which a drug is intended to be used or has been approved to treat.
indolent: Slow-growing or slow-progressing. An indolent disease advances gradually and may not need aggressive treatment right away.
indolent systemic mastocytosis: The most common form of systemic mastocytosis, characterized by slow progression and manageable symptoms with medication.
induction therapy: The first phase of treatment, designed to rapidly reduce the amount of disease and achieve remission.
inebilizumab: An anti-CD19 monoclonal antibody (brand name Uplizna) that depletes B cells. Approved for neuromyelitis optica spectrum disorder, IgG4-related disease and, since December 2025, generalized myasthenia gravis in adults with AChR or MuSK antibodies.
infantile fibrosarcoma: A rare soft-tissue tumor occurring in infants and young children that frequently harbors NTRK gene fusions.
infarction: Death of tissue caused by a blocked blood vessel cutting off the blood supply. A heart attack is a myocardial infarction.
infigratinib: An FGFR-blocking pill originally approved for bile duct cancer, now being developed in low doses for achondroplasia.
inflammation: The body's protective response to injury or infection, causing redness, swelling, heat, and pain. Chronic inflammation can damage tissues.
infliximab: An anti-inflammatory antibody (brand Remicade) that blocks TNF, used in Crohn's disease, rheumatoid arthritis, and other autoimmune conditions.
informed consent: A process where you learn about a study's risks and benefits before agreeing to participate.
infusion: Slowly delivering a medication or fluid directly into a vein over a period of time.
infusion reaction: An adverse response that can occur during or shortly after an IV infusion, such as fever, chills, flushing, or difficulty breathing. Usually manageable with pre-medications.
INO-3107: INOVIO's DNA immunotherapy for adults with recurrent respiratory papillomatosis, given as 4 injections into muscle, with an FDA decision due October 30, 2026.
inotersen: An older antisense TTR silencer (brand name Tegsedi) given weekly under a restricted program. Listed by the FDA as discontinued in the United States.
INR: International Normalized Ratio — measures how quickly your blood clots.
insidious: Developing gradually and without obvious symptoms at first, making the condition harder to detect early.
INSPIRE trial: The Phase 2/3 trial (NCT05397665) of govorestat (AT-007) in patients with CMT-SORD, run by Applied Therapeutics. Listed as completed in June 2026.
insulin: A hormone produced by the pancreas that allows cells to absorb glucose (sugar) from the blood for energy.
intellectual disability: Significant limitations in cognitive functioning and adaptive behaviors that develop before adulthood.
intention-to-treat: An analysis that includes all patients as originally assigned, even if they dropped out or switched groups, giving a real-world picture of treatment effect.
intercellular: Between cells. Intercellular junctions and signals allow neighboring cells to communicate and hold together.
interferon: Proteins produced by immune cells that help fight viral infections and regulate the immune response. Also used as medications.
interim analysis: A planned look at study data before the trial is finished, to check for early signs of benefit or harm.
internal appeal: Asking your insurer to review its own denial. Employer and marketplace plans must decide within 30 days for a drug you have not received, 60 days for one you have, or 72 hours if urgent.
interstitial lung disease: A group of more than 200 conditions that inflame or scar the tissue around the lungs' air sacs, making it harder to get oxygen into the blood. IPF is the most common scarring form.
interval extension: Spacing out drug doses farther apart (for example, from every 4 weeks to every 6-8 weeks) while maintaining disease control.
intracellular: Inside cells. Intracellular processes include metabolism, protein production, and energy generation.
intracerebral: Within the brain tissue itself, as opposed to the surrounding membranes or fluid spaces.
intracerebroventricular: Delivered directly into the fluid-filled spaces of the brain through a surgically implanted port. Used for Brineura in Batten disease.
intracranial: Inside the skull. Intracranial pressure refers to the pressure within the skull that can affect the brain.
intractable: Extremely difficult to treat or manage. Intractable pain or seizures do not respond to standard therapies.
intraepidermal: Within the epidermis, the outermost layer of the skin.
intrahepatic: Within the liver. Intrahepatic conditions affect the liver tissue or bile ducts inside the liver.
intramural: Within the wall of a hollow organ, such as the heart wall or intestinal wall.
intramuscular: Injected into a muscle, a common route for delivering vaccines and certain medications.
intranasally: Administered through the nose, such as nasal sprays or inhaled medications.
intraperitoneal: Within the abdominal cavity, the space containing the stomach, intestines, liver, and other organs.
intrathecal: Delivered directly into the fluid around the spinal cord.
intrauterine: Inside the uterus (womb). Can refer to conditions affecting a developing fetus or to certain medical devices.
intravascular: Inside the blood vessels. Intravascular treatments are delivered directly into the bloodstream.
intravenous: Given directly into a vein through an IV line.
intron: Non-coding segments of a gene between exons. Usually removed during protein production, but mutations here can still disrupt the process.
intubation: Placing a tube through the mouth into the airway to help a patient breathe with a ventilator.
investigational: Relating to a drug or treatment that is being tested but not yet approved.
iptacopan: An oral complement factor B inhibitor (brand name Fabhalta). Approved in adults for paroxysmal nocturnal hemoglobinuria, for slowing kidney function decline in primary IgA nephropathy, and for reducing proteinuria in C3 glomerulopathy.
IRB: Institutional Review Board — a committee that reviews and approves clinical research to ensure participant safety and rights.
ischemia: Insufficient blood flow to a tissue or organ, which can cause damage.
Isembyld: The brand name for apitegromab, an antibody that blocks myostatin to improve muscle function in spinal muscular atrophy. Given by infusion every 4 weeks as an add-on to Spinraza, Evrysdi or Zolgensma.
IV infusion: A medication delivered directly into a vein through a drip line, typically at a clinic or infusion center over a period of hours.
IVIG: Intravenous immunoglobulin, antibodies pooled from thousands of blood donors and given by IV. Doctors use it for myasthenia gravis flares and sometimes as ongoing treatment, often off label.
Ixodes: A genus of hard-bodied ticks that transmit Lyme disease, babesiosis, and anaplasmosis. Includes the blacklegged tick (I. scapularis) and western blacklegged tick (I. pacificus).

J

JAK2: A signaling enzyme that tells the bone marrow to make blood cells. The JAK2 V617F mutation drives polycythemia vera and other myeloproliferative neoplasms.
Jakavi: Brand name for ruxolitinib (known as Jakafi in the US), a JAK inhibitor approved for myelofibrosis and polycythemia vera.
Jarisch-Herxheimer: A temporary worsening of symptoms when large numbers of bacteria or parasites die during treatment, releasing inflammatory byproducts.
JC virus: A common virus most adults carry without symptoms. In people with a weakened immune system it can cause PML, a serious brain infection.
jejunum: The middle section of the small intestine, where most nutrient absorption takes place.
joint contracture: Permanent shortening of muscles or connective tissue limiting joint movement.

K

Karnofsky: A score (0-100) measuring how well you function day-to-day. 100 = no complaints.
Kayser-Fleischer ring: Greenish rings visible at the edge of the cornea, a diagnostic sign of Wilson disease.
KCNT1: A gene for a potassium channel in brain cells. KCNT1 mutations cause severe early-childhood epilepsies.
kDa: Kilodaltons; a unit measuring protein size. Used to identify specific bands on Western blots.
KDIGO: Kidney Disease Improving Global Outcomes, the international body that writes the treatment guidelines most kidney specialists follow. Its IgA nephropathy guideline was last updated in 2025.
keratitis: Inflammation of the cornea (the clear front surface of the eye), causing pain, redness, and blurred vision.
ketoacidosis: A serious complication where the body produces excess blood acids (ketones), most commonly in uncontrolled diabetes.
ketogenic: A diet or metabolic state where the body burns fat for fuel instead of carbohydrates, producing ketone bodies.
Keytruda: Brand name for pembrolizumab, a PD-1 immune checkpoint inhibitor approved for multiple cancers including rare types like Merkel cell carcinoma and mesothelioma.
kinase resistance mutation: A new mutation in cancer cells that prevents a targeted drug from binding, causing the cancer to stop responding to treatment.
kinky hair: Sparse, coarse, twisted hair with a steel-wool texture — a characteristic feature of Menkes disease caused by copper deficiency affecting hair keratin.
KIT D816V: The most common genetic mutation driving systemic mastocytosis. It keeps the KIT protein permanently switched on, causing uncontrolled mast cell growth.
KIT inhibitor: A targeted drug that blocks the KIT protein, stopping the uncontrolled mast cell growth driven by KIT mutations.
KLKB1: The gene for prekallikrein, a protein in the swelling pathway behind hereditary angioedema. Silencing KLKB1 is one strategy to prevent attacks.
knockout: A genetic engineering technique where a gene is deliberately inactivated to study its function or model a disease.
Kv7: A family of voltage-gated potassium channels in nerve cells. When opened, they stabilize electrical activity and reduce excessive nerve firing. Drug targets for epilepsy and pain conditions.
kyphosis: An excessive forward rounding of the upper back, sometimes called a hunchback.

L

lactate: A substance that builds up in blood or spinal fluid when cells cannot make energy with oxygen efficiently. A raised level is a clue to mitochondrial disease, though a normal level does not rule it out.
lanadelumab: A monoclonal antibody (brand name Takhzyro) that blocks plasma kallikrein to prevent hereditary angioedema attacks.
lanreotide: A long-acting somatostatin analog (brand Somatuline) injected monthly to control acromegaly and neuroendocrine tumors.
laronidase: An enzyme replacement therapy (brand name Aldurazyme) given by IV infusion to supply the missing enzyme in MPS I. Does not treat brain symptoms.
larotrectinib: A TRK inhibitor (brand name Vitrakvi) approved for cancers with NTRK gene fusions, regardless of tumor type.
laryngeal: Relating to the larynx (voice box), the structure in the throat containing the vocal cords.
late-onset Pompe: A form of Pompe disease where symptoms begin in childhood, adolescence, or adulthood — typically milder and more slowly progressive than the infantile form.
LCA10: Leber congenital amaurosis type 10, a severe inherited retinal disease present from infancy, caused by CEP290 gene mutations.
LEAP2MONO: Sanofi's Phase 3 trial of venglustat against enzyme infusions in 43 people aged 12 and older with Gaucher disease type 3. A combined balance and thinking score favored venglustat (p=0.0179).
Lee symptom scale: A validated questionnaire used to measure symptom burden in chronic graft-versus-host disease across seven domains (skin, eyes, mouth, lungs, nutrition, energy, psych).
left ventricular hypertrophy: Thickening of the heart's main pumping chamber wall, which makes the heart work harder. A common complication of Fabry disease and acromegaly.
left ventricular mass: The weight of the heart's main pumping chamber muscle. An elevated measurement indicates the heart wall is thickening, which can affect heart function.
Leigh syndrome: A rare inherited mitochondrial disease that starves deep-brain regions of energy, usually starting in the first year of life. As of 2026 it has no FDA-approved treatment.
lenalidomide: An oral myeloma drug (brand Revlimid) from the immunomodulator class, used at nearly every stage of treatment and in many trial combinations.
lentivirus: A type of virus used in gene therapy that can insert its genetic material into the patient's DNA for long-lasting effect.
lenvatinib: A cancer pill that blocks several growth signals, including those that feed new blood vessels, being tested with pembrolizumab in RRP.
lesion: An area of abnormal tissue found through examination or imaging.
letter of medical necessity: A letter from your prescriber explaining why a specific drug is needed for you: diagnosis, prior treatments, the label indication and the harm of delay. The core of most successful appeals.
leukocyte: White blood cell — part of your immune system.
leukodystrophy: A group of genetic disorders that damage the white matter of the brain, progressively destroying the myelin that insulates nerve fibers.
leukoencephalopathy: Disease of the brain's white matter, which contains the nerve fibers that connect different brain regions.
leukopenia: An abnormally low white blood cell count, which weakens the body's ability to fight infection.
levacetylleucine: A modified form of the amino acid leucine, sold as Aqneursa, that improves cerebellar ataxia. Its exact target is unknown; it is thought to normalize energy metabolism in brain cells.
Lewy body: Abnormal protein deposits found in nerve cells in Parkinson disease and related disorders.
LGMD2I: Limb-girdle muscular dystrophy type 2I (also called LGMD R9), a muscle-weakening disease caused by FKRP gene mutations.
life expectancy: How long a person is expected to live based on their current health.
likely pathogenic: A genetic variant with strong but not complete evidence that it causes disease. Doctors act on it much as they do on a pathogenic variant.
limited distribution: An arrangement where a drugmaker contracts with only 1 or 2 specialty pharmacies to dispense a drug nationally. Your local pharmacy cannot order it; prescriptions route through the contracted pharmacy instead.
linsitinib: An experimental pill from Sling Therapeutics that blocks IGF-1R, the same receptor targeted by Tepezza, an IV infusion. It is in Phase 3 testing for thyroid eye disease.
lipidosis: A group of inherited metabolic disorders where fats (lipids) accumulate abnormally in cells due to missing enzymes.
lipofuscin: A yellowish-brown waste pigment that builds up inside cells when they cannot break it down. It collects in the retina in Stargardt disease, where it is toxic to photoreceptors and drives vision loss, and in nerve cells in neuronal ceroid lipofuscinosis.
lipofuscinosis: Accumulation of lipofuscin (a yellowish-brown waste pigment) inside cells, particularly nerve cells, causing progressive damage.
lirafugratinib: A pill that selectively blocks FGFR2, approved in September 2026 as Lyrfigtu for previously treated bile duct cancer with an FGFR2 fusion.
Lisch nodule: Harmless, pigmented bumps on the iris of the eye. Present in over 90% of adults with NF1 and useful for diagnosis.
liver biopsy: A small piece of liver tissue removed (usually with a needle) and analyzed in a lab. In Wilson disease, used to measure copper concentration directly in liver tissue.
liver enzyme: Proteins such as ALT and AST that leak into the blood when liver cells are stressed or damaged. Small, short-lived rises are common with many drugs.
LLN: Lower Limit of Normal — the lowest value considered normal for a lab test.
loading dose: A larger starting dose (or series of doses) used to build the drug up to effective levels quickly, before settling into a smaller maintenance dose.
locally advanced: Cancer that has grown beyond its original site but hasn't spread to distant organs.
lone star tick: Amblyomma americanum, a tick species native to the southeastern and south-central United States with a distinctive single white spot on the female. The primary vector for triggering Alpha-Gal Syndrome through its bite.
LOPD: Late-onset Pompe disease — the form of Pompe disease that appears after infancy, typically affecting muscles and breathing gradually over years.
loss of function: A mutation that reduces or eliminates the normal activity of a protein, causing disease.
LOTS trial: Late-Onset Treatment Study — the pivotal Phase III trial that led to Lumizyme's approval for late-onset Pompe disease.
lower motor neuron sign: Clinical findings indicating damage to nerve fibers in the spinal cord or peripheral nerves.
LRP4: A protein that helps nerve signals reach muscle. Antibodies against LRP4 cause some cases of myasthenia gravis.
lumbar puncture: A procedure where cerebrospinal fluid is collected from the lower spine using a needle.
lung density: A CT-scan measure of how much tissue the lungs contain. It falls as emphysema destroys lung tissue, so a slower decline means slower disease.
lupus: An autoimmune disease where the immune system attacks many different tissues, causing inflammation in the joints, skin, kidneys, and other organs.
luspatercept: An erythroid maturation agent (brand name Reblozyl) approved for anemia in beta thalassemia and myelodysplastic syndromes.
Luxturna: The brand name for voretigene neparvovec, the first FDA-approved gene therapy for an inherited retinal disease (RPE65 mutations).
LVEF: Left Ventricular Ejection Fraction — measures how well your heart pumps blood. Normal is 55-70%.
LVH: Left ventricular hypertrophy — thickening of the heart's left ventricle wall, often caused by conditions like Fabry disease or high blood pressure.
LVOT: Left ventricular outflow tract, the channel blood passes through as it leaves the heart's main pumping chamber for the aorta.
lymph node dissection: Surgical removal of lymph nodes to check for or remove cancer that has spread.
lymphadenitis: Inflammation and swelling of lymph nodes, usually caused by infection.
lymphadenopathy: Swollen lymph nodes, which can be a sign of infection, autoimmune disease, or cancer.
lymphangioleiomyomatosis: A rare lung disease primarily affecting women, where abnormal smooth muscle cells grow in the lungs, blocking airways and blood vessels.
lymphocytosis: An abnormally high number of lymphocytes (a type of white blood cell) in the blood.
lymphopenia: An abnormally low number of lymphocytes in the blood, weakening the immune system's ability to fight infections.
lymphoplasmacytic: A type of lymphoma composed of both lymphocytes and plasma cells, as seen in Waldenstrom's macroglobulinemia.
Lyrfigtu: The brand name of lirafugratinib, a pill built to block FGFR2 more selectively than older FGFR drugs, FDA-approved on September 23, 2026 for bile duct cancer with an FGFR2 fusion.
lyso-Gb3: A breakdown product of Gb3 measured in blood tests. An important biomarker for tracking Fabry disease activity and treatment response.
lysophosphatidic acid: A fat-based signaling molecule that tells cells to move and lay down scar tissue. Its receptor LPA1 is the target of admilparant in pulmonary fibrosis.
lysosome: Cellular compartments that contain enzymes for breaking down and recycling cellular waste.
Lytgobi: Brand name for futibatinib, an FGFR-blocking pill approved for intrahepatic bile duct cancer with an FGFR2 fusion.

M

M protein: The single abnormal antibody fragment that myeloma cells pour into the blood. Doctors measure it to diagnose myeloma and to track whether treatment is working.
macroadenoma: A pituitary tumor larger than 1 centimeter. Most growth-hormone tumors are already this size when acromegaly is diagnosed, which makes complete surgical removal harder.
macrocephaly: An abnormally large head size, which can indicate fluid buildup or abnormal brain growth.
macroglobulinemia: An excess of IgM, the largest antibody protein, in the blood, which can thicken it and slow circulation.
macrophage: White blood cells that engulf and digest bacteria, dead cells, and other debris.
macrophage activation syndrome: A life-threatening complication of sJIA and other inflammatory conditions where immune cells become overactivated, causing a cytokine storm with organ failure. Also called secondary HLH.
macular: The central area of the retina responsible for sharp, detailed vision used for reading and recognizing faces.
maintenance therapy: Ongoing lower-intensity treatment given after initial therapy to keep the disease in remission and prevent relapse.
MAJESTY: The Phase 3 trial comparing Gazyva with tacrolimus in 142 adults with primary membranous nephropathy. At 2 years, 37% on Gazyva were in complete remission against 6% on tacrolimus.
major amendment: A large batch of new information a company sends the FDA in the middle of a review. The FDA can respond by pushing its decision deadline back 3 months; adrabetadex, Molbreevi and deramiocel were all extended this way in 2026.
malabsorption: The inability of the intestines to properly absorb nutrients from food, leading to nutritional deficiencies.
malignant: Cancerous. A malignant tumor can invade nearby tissue and spread to other parts of the body.
mammalian meat allergy: Allergy to red meat (beef, pork, lamb, venison) caused by IgE antibodies to alpha-gal. The clinical presentation of Alpha-Gal Syndrome.
mammalian-derived: Made from the tissue of mammals such as pigs, cattle, or sheep. In alpha-gal syndrome these ingredients matter because mammal tissue carries the alpha-gal sugar.
managed access program: A company-run route to receive a drug that is still under FDA review, outside a clinical trial, for patients who cannot wait. Novartis has opened one for del-zota.
mannosidosis: A rare inherited disorder (alpha-mannosidosis) where a missing enzyme lets sugar fragments build up in cells, affecting hearing, immunity, and development.
mantle cell lymphoma: A rare, aggressive type of non-Hodgkin lymphoma that develops from B cells in the outer edge (mantle zone) of a lymph node.
MAR002: Marea Therapeutics' long-acting antibody that blocks the growth hormone receptor, designed for dosing every 2 weeks to monthly instead of the daily injections of pegvisomant.
marstacimab: The generic name of Hympavzi, a weekly anti-TFPI antibody injected under the skin to prevent bleeds in hemophilia A or B from age 6, with or without inhibitors.
MAS: Macrophage Activation Syndrome — a life-threatening hyperinflammatory crisis that can complicate sJIA, where immune macrophages become overactivated.
MASC: Mammary analogue secretory carcinoma, the earlier name for secretory carcinoma of the salivary gland, a tumor defined by the ETV6-NTRK3 fusion.
masitinib: An experimental pill from AB Science that blocks several tyrosine kinases, enzymes that act as on switches inside cells, including immune cells such as mast cells and microglia. It is in Phase 3 testing for ALS and is not approved in the US.
mast cell: Immune cells found throughout your body that release histamine and other chemicals. In mastocytosis, they multiply abnormally and cause symptoms.
mast cell degranulation: When mast cells suddenly release their stored chemicals (histamine, tryptase), causing flushing, itching, GI symptoms, or anaphylaxis.
mast cell mediator: Chemical messengers (histamine, tryptase, heparin) stored inside mast cells and released during reactions, causing allergy-like symptoms.
mast cell stabilizer: A medication that prevents mast cells from releasing their chemical mediators, reducing symptom episodes.
mastocytosis: A rare condition caused by an overproduction of mast cells (immune cells) that accumulate in skin and/or internal organs, triggering allergic-type reactions.
maximum tolerated dose: The highest dose of a drug that can be given without causing unacceptable side effects.
MC1R: Melanocortin 1 receptor, a protein on pigment cells that controls melanin production. Drugs that activate it increase skin pigment, which can shield people with light-sensitivity disorders like protoporphyria.
MC4R: A brain receptor that helps regulate hunger and body weight. Drugs targeting the MC4R pathway treat rare genetic obesity disorders.
MCT8: A transporter protein that carries thyroid hormone into cells, including brain cells. MCT8 deficiency causes a rare disorder of brain development.
MCT8 deficiency: A rare X-linked disorder, also called Allan-Herndon-Dudley syndrome, in which the MCT8 transporter is missing so thyroid hormone cannot enter the brain while it builds up in the blood. Causes severe motor and intellectual disability in boys.
measurable disease: Disease that can be seen and measured on imaging scans.
mechanical ventilation: Use of a breathing machine to assist or replace normal breathing.
mechanism of action: The way a drug works in the body to produce its therapeutic effect.
MeCP2: Methyl-CpG-binding protein 2 — a protein essential for brain development. Mutated in Rett syndrome.
median survival: The length of time by which 50% of patients with a disease are expected to survive.
Medicare Advantage: Private health plans approved by Medicare that replace Original Medicare's Part A and B coverage, usually with a network, and cap what you pay each year for in-network medical care.
Medicare Part B: The part of Medicare that covers doctor visits, outpatient care and drugs a clinician gives you, like infusions in a doctor's office. You usually pay 20% after the deductible.
Medicare Part D: Medicare's prescription drug coverage for medicines you fill at a pharmacy, bought through a private drug plan or a Medicare Advantage plan. In 2027 your yearly out-of-pocket cost is capped at $2,400.
Medicare Prescription Payment Plan: A free, voluntary Medicare option that spreads your Part D out-of-pocket costs into monthly bills across the year. It changes when you pay, not how much.
Medigap: Private supplemental insurance that works with Original Medicare to pay some or all of the costs Medicare leaves to you, such as the 20% coinsurance on Part B services.
MEK inhibitor: A class of drugs that block MEK, a protein in the RAS/MAPK signaling pathway. Used to shrink plexiform neurofibromas in NF1.
membranous nephropathy: A kidney disease in which antibodies attack the kidney's filters, causing protein to leak into the urine. Anti-CD20 and anti-CD19 drugs are used or studied for it.
Mendelian disorder: A genetic disease following predictable inheritance patterns from a single gene.
meningitis: Inflammation of the protective membranes (meninges) surrounding the brain and spinal cord, often caused by infection.
meningococcal: Caused by Neisseria meningitidis, a bacterium behind meningitis and bloodstream infections that can turn deadly within hours. Drugs that block complement C5 raise this risk sharply, so vaccines are required before starting them.
mepolizumab: A monoclonal antibody (brand name Nucala) that blocks interleukin-5, reducing eosinophil levels. FDA-approved for severe eosinophilic asthma and EGPA (2017).
Merkel cell carcinoma: A rare, aggressive skin cancer often linked to the Merkel cell polyomavirus or UV exposure. It tends to grow quickly and can spread to lymph nodes.
mesangial: Relating to the mesangium, the tissue that supports the tiny blood vessel clusters (glomeruli) that filter your blood in the kidneys.
mesangium: The supportive tissue between the tiny blood filtering units in the kidneys. In IgA nephropathy, abnormal antibodies deposit here and cause damage.
mesenchymal stem cell: A type of adult stem cell that can be grown in laboratories. Mesenchymal stem cells generally help nearby cells through signaling molecules they release (paracrine effects), rather than by replacing damaged cells directly.
mesothelium: The thin membrane lining body cavities like the chest and abdomen. Mesothelioma is a cancer of this tissue.
metabolic acidosis: Abnormally high acidity in the blood due to buildup of acids or loss of bases.
metabolic decompensation: A dangerous episode where the body's metabolic balance suddenly breaks down, often triggered by illness, fasting, or stress. In MMA, this causes rapid buildup of toxic acids.
metabolic disease specialist: A doctor specializing in inherited metabolic disorders — conditions where the body cannot properly process certain substances due to enzyme deficiencies.
metabolite: A substance produced when the body breaks down food, drugs, or chemicals. Some metabolites are toxic if they accumulate.
metastatic: Cancer that has spread from its original location to other parts of the body.
methotrexate: A chemotherapy drug that blocks a vitamin cells need to divide. Given at very high doses into a vein, it crosses into the brain and is the backbone of first treatment for primary central nervous system lymphoma.
methylation: A chemical process in which a methyl group is attached to DNA or to a protein. On DNA it can switch genes on or off without changing the underlying sequence, which is one way cells with identical genes end up doing different jobs.
methylmalonic acidemia: An inherited metabolic disorder where the body cannot properly break down certain proteins and fats, causing toxic methylmalonic acid to accumulate.
methylmalonyl-CoA mutase: The enzyme that converts methylmalonyl-CoA to succinyl-CoA in a key metabolic pathway. Deficiency of this enzyme causes the most severe form of methylmalonic acidemia.
MFN2: Mitofusin 2, a gene that codes for a protein involved in mitochondrial function and nerve fiber health. Mutations cause CMT2A.
MG-ADL: Myasthenia Gravis Activities of Daily Living, a 0 to 24 score built from 8 questions about talking, chewing, swallowing, breathing, brushing teeth or combing hair, getting up from a chair, double vision and drooping eyelids. Lower is better, and a drop of 2 points or more is usually counted as an improvement patients notice.
MGUS: Monoclonal gammopathy of undetermined significance, a silent condition in which a blood test finds an abnormal antibody protein but there is no cancer. A small share of people with MGUS go on to develop multiple myeloma, so it is monitored rather than treated.
micro-dystrophin: A shorter, engineered version of the dystrophin protein used in gene therapy to restore partial muscle protection.
microadenoma: A pituitary tumor smaller than 1 centimeter. Surgery cures most of them when done by an experienced pituitary surgeon.
microangiopathy: Disease of the small blood vessels, causing them to thicken, weaken, or become blocked.
microcephaly: An abnormally small head size, often indicating that the brain has not developed fully.
microdebrider: A small surgical tool with a rotating blade and suction, used to shave papillomas off the vocal cords and airway.
microthrombocytopenia: A condition with abnormally small and few platelets, increasing the risk of both bleeding and clotting problems.
midostaurin: A multi-kinase inhibitor (brand name Rydapt) approved for advanced systemic mastocytosis and certain leukemias.
migalastat: An oral chaperone therapy (brand name Galafold) for Fabry disease that stabilizes the faulty enzyme so it can function better. Works only for patients with specific amenable mutations.
Million Veteran Program: A VA research program that links genetic data to the health records of hundreds of thousands of veterans. Its 2026 study of 2,658 V122I carriers found only 3.3% had been diagnosed with amyloidosis.
mineralocorticoid: A type of steroid hormone that regulates salt and water balance in the body, affecting blood pressure and fluid levels.
mirdametinib: An oral MEK inhibitor (brand name Koselugo alternative) approved for NF1-related plexiform neurofibromas. Works by blocking the overactive RAS/MAPK signaling pathway.
missense mutation: A change in DNA that causes a protein to be produced with a different amino acid, potentially affecting function.
mitapivat: An oral pill from Agios that activates pyruvate kinase, an enzyme red blood cells use for energy. It is sold as Pyrukynd and Aqvesme and is under FDA review for sickle cell disease.
mitral valve: The heart valve between the left atrium and left ventricle that prevents blood from flowing backward.
MoDE: Molecular Degrader of Extracellular proteins, Biohaven's platform, licensed from Yale, for drugs that attach to a disease-causing protein such as an IgG antibody in the blood and deliver it to the liver to be destroyed. BHV-1300 for Graves' disease is the first MoDE drug in a Phase 3 trial.
MOGAD: MOG antibody-associated disease, a rare inflammatory condition of the optic nerves, spinal cord and brain driven by antibodies against myelin oligodendrocyte glycoprotein. It is related to but distinct from MS and NMOSD.
monoclonal antibody: A manufactured antibody that targets specific proteins to help the immune system fight disease.
monogenic: Caused by a mutation in a single gene, as opposed to conditions involving multiple genes.
monosomy: Having only one copy of a chromosome instead of the normal two. Turner syndrome is caused by monosomy X.
monotherapy: Treatment using a single drug.
morbidity: The burden of disease, including illness, complications, and reduced quality of life associated with a condition.
mortality: The death rate from a disease in a given population over a specific time period.
motor neuron: A nerve cell that carries signals from the brain and spinal cord to muscles.
MPNST: Malignant Peripheral Nerve Sheath Tumor — a rare, aggressive cancer that can arise from plexiform neurofibromas in NF1 patients. Lifetime risk is 8–13%.
MPS I: Mucopolysaccharidosis Type I, an inherited disorder caused by alpha-L-iduronidase enzyme deficiency. Ranges from severe (Hurler) to mild (Scheie) forms.
MRI: Magnetic Resonance Imaging — a scan using magnetic fields to create detailed images of internal structures.
mRNA therapy: An emerging treatment approach that delivers messenger RNA to cells, instructing them to produce a missing or deficient protein. Being studied for MMA and other metabolic diseases.
MTC: Medullary Thyroid Cancer — a rare cancer arising from thyroid C cells. About 25% of cases are hereditary due to RET gene mutations.
MTHFR: Methylenetetrahydrofolate reductase; a gene involved in methylation and detoxification. Common variants (C677T, A1298C) may affect how the body processes treatment metabolites.
MTTT: Modified Two-Tier Testing; an FDA-cleared alternative to the Western blot for Lyme diagnosis. Uses two sequential blood tests with better sensitivity for early infection.
mucosal: Relating to mucous membranes, the moist tissue lining body cavities like the mouth, nose, lungs, and digestive tract.
multi-hit: A disease model where multiple biological steps must go wrong in sequence for the disease to develop, as in IgA nephropathy.
multi-kinase inhibitor: A drug that blocks multiple growth-signaling proteins at once. Effective but may cause more side effects than selective inhibitors.
multi-organ failure: When 2 or more major organs, such as the liver, kidneys or lungs, stop working properly at the same time. It is life threatening.
multicenter: A clinical study conducted at multiple hospitals or research sites, which helps ensure results are broadly applicable.
multidisciplinary: Involving specialists from multiple medical fields working together to provide the best care for complex conditions.
muscle biopsy: A procedure where a small piece of muscle tissue is removed and examined under a microscope, often used in gene therapy trials to check whether the treatment is producing the target protein.
muscular dystrophy: A group of inherited diseases that cause progressive weakness and degeneration of skeletal muscles. Duchenne is the most common severe form.
MuSK: Muscle-specific kinase, a protein that holds acetylcholine receptors in place on the muscle. About 5% to 8% of people with generalized myasthenia gravis make antibodies against MuSK instead of AChR, and none of the complement inhibitors are approved for them.
MUT gene: The gene encoding methylmalonyl-CoA mutase. Mutations in MUT cause methylmalonic acidemia, with mut0 (no enzyme activity) being more severe than mut− (reduced activity).
mut−: A classification of MMA where the methylmalonyl-CoA mutase enzyme has some residual activity but less than normal. Generally milder than mut0.
mut0: A classification of MMA where the methylmalonyl-CoA mutase enzyme has zero residual activity. This is the most severe form of MMA.
mutation: A change in a gene's DNA sequence that may affect how a protein works.
myasthenic crisis: A life-threatening complication of myasthenia gravis where respiratory muscles weaken to the point of requiring mechanical ventilation.
MYBPC3: A gene for a heart muscle protein. MYBPC3 mutations are the most common genetic cause of hypertrophic cardiomyopathy.
Mycapssa: Oral octreotide capsules taken twice daily, approved in 2020 for adults with acromegaly already controlled on octreotide or lanreotide injections.
mycophenolate: An immune-suppressing pill (brand CellCept) originally used in transplant patients and now used off label to prevent attacks in MOGAD and other autoimmune diseases.
myelin: The fatty insulation around nerve fibers that helps electrical signals travel quickly.
myelitis: Inflammation of the spinal cord, which can cause weakness, numbness, and loss of bowel or bladder control.
myelodysplasia: Faulty blood cell production by the bone marrow, as in myelodysplastic syndromes, which can progress toward leukemia.
myelodysplastic: Relating to a group of disorders where the bone marrow produces abnormal blood cells that do not function properly.
myelofibrosis: A rare blood cancer in which scar tissue builds up inside the bone marrow, crowding out normal blood cell production.
myeloma: A cancer of plasma cells (antibody-producing white blood cells) in the bone marrow.
myeloproliferative: Relating to conditions where the bone marrow overproduces certain types of blood cells.
myelosuppression: Reduced bone marrow activity leading to lower blood cell counts, a common side effect of chemotherapy.
MYH7: The gene for the main motor protein in heart muscle (beta-myosin heavy chain). With MYBPC3, one of the 2 most common genetic causes of hypertrophic cardiomyopathy.
myocarditis: Inflammation of the heart muscle, often caused by a viral infection, which can weaken the heart's pumping ability.
myoclonus: Sudden, involuntary jerking or twitching of muscles.
myopia: Nearsightedness — distant objects appear blurry while close objects are clear.
myosin inhibitor: A drug that dials down myosin, the motor protein that makes heart muscle contract, so an overactive thickened heart squeezes less hard.
myositis: Inflammation of the muscles, causing weakness and sometimes pain, often from autoimmune causes.
myostatin: A protein the body makes to limit how much muscle it builds, acting as a natural brake on muscle growth. Blocking it is a way to try to build muscle in wasting diseases.

N

Naglazyme: Brand name for galsulfase, an enzyme replacement therapy approved for mucopolysaccharidosis type VI (MPS VI, Maroteaux-Lamy syndrome). Made by BioMarin.
nanobody: An antibody fragment about a tenth the size of a full antibody, made of a single binding piece first found in camels and llamas. Its small size suits low-volume injections, as with gefurulimab.
natural history: How a disease progresses on its own, without treatment. Natural history studies document that course and often serve as the comparison for rare disease trials.
natural history study: Research that tracks how a disease progresses without intervention.
natural killer cell: A type of immune cell that can recognize and destroy virus-infected cells and cancer cells without prior exposure.
navepegritide: The generic name of Yuviwel, a once-weekly shot for children with achondroplasia. It is CNP attached to a slow-release carrier, so one dose keeps working for a week instead of a day.
NavigATTR: AstraZeneca's no-charge TTR genetic testing program for US adults, ordered by a cardiology, genetics or neurology clinician.
NCL: Neuronal Ceroid Lipofuscinosis, the scientific name for Batten disease. Different types (CLN1-14) affect different enzymes.
NDA: New Drug Application — the formal submission to the FDA requesting permission to market a new drug.
nebulizer: A device that turns liquid medicine into a fine mist that is breathed in through a mouthpiece or mask.
necrosis: Death of body tissue, often caused by loss of blood supply, infection, or injury.
Nefecon: Calliditas' name for the delayed-release budesonide capsule sold in the US as Tarpeyo, built to release the steroid in the lower small intestine where the faulty IgA1 is made.
NefIgArd trial: The Phase III clinical trial of TARPEYO (budesonide delayed-release) in IgA nephropathy.
neoadjuvant: Treatment given before the main treatment (usually surgery) to shrink the disease first.
neoadjuvant therapy: Treatment given before the main treatment (usually surgery) to shrink a tumor and make it easier to remove.
neoplasia: Abnormal and uncontrolled cell growth that may be benign (harmless) or malignant (cancerous).
neoplasm: An abnormal growth of tissue, which may be benign or cancerous.
nephritic: Relating to inflammation of the kidneys, typically involving blood in the urine and reduced kidney function.
nephritis: Inflammation of the kidneys, which can impair their filtering function and lead to kidney damage.
nephrogenic: Originating from or caused by the kidneys.
nephrolithiasis: The formation of kidney stones, hard mineral deposits that can cause severe pain when passing through the urinary tract.
nephrologist: A doctor specializing in kidney diseases. Manages IgA nephropathy, Fabry disease kidney involvement, and other kidney conditions.
nephropathic cystinosis: The most common and most severe form of cystinosis, an inherited disease in which the amino acid cystine builds up inside cells. It starts in infancy and damages the kidneys first, then the eyes and other organs.
nephropathy: Disease of the kidneys that affects their ability to filter waste from the blood.
nephrotic: Relating to a condition where the kidneys leak large amounts of protein into the urine.
nephrotic syndrome: A kidney condition causing severe protein loss in urine, low blood protein, high cholesterol, and significant swelling.
nephrotoxic: Capable of causing damage to the kidneys. Kidney function is monitored when using nephrotoxic drugs.
nerve biopsy: Removal of a small piece of nerve tissue (usually from the sural nerve in the lower leg) for laboratory examination. Used in unusual cases of peripheral neuropathy when genetic testing is inconclusive.
nerve conduction: A test that measures how fast electrical impulses move along a nerve.
nerve conduction study: A diagnostic test that measures how fast electrical signals travel through your nerves, done by stimulating the nerve at one point and recording the response at another. Used to diagnose CMT and other peripheral neuropathies.
net price: What an insurer actually pays for a drug after the manufacturer's discounts and rebates, which is often lower than the list price.
neuritis: Inflammation of a nerve or group of nerves, causing pain, tingling, numbness, or weakness.
neuroblastoma: A cancer of immature nerve cells, most often affecting young children, usually starting in the adrenal glands or along the spine.
neuroborreliosis: Lyme disease affecting the nervous system, which can cause facial weakness, nerve pain, and inflammation around the brain.
neurodegeneration: Progressive loss of structure and function of neurons in the nervous system.
neurodegenerative: Relating to progressive loss of nerve cell structure and function.
neurodegenerative disease: A progressive disorder where nerve cells gradually deteriorate and lose function over time.
neurodevelopmental: Relating to the growth and development of the brain and nervous system. Neurodevelopmental disorders affect learning, behavior, and movement.
neuroendocrine: Relating to cells that release hormones in response to nerve signals. Medullary thyroid cancer is a neuroendocrine tumor.
neurofibromatosis: A group of genetic disorders that cause tumors to grow along nerves, most commonly neurofibromatosis type 1 (NF1).
neurofibromin: A tumor-suppressor protein produced by the NF1 gene. Loss of neurofibromin activates the RAS/MAPK pathway, driving tumor growth in neurofibromatosis type 1.
neurofilament: Structural proteins inside nerve cells that provide support. Elevated neurofilament levels in blood indicate nerve damage.
neurofilament light chain: A protein that leaks from damaged nerve cells; elevated levels indicate neuronal damage.
neurogenic: Caused by or originating from the nervous system.
neuroinflammation: Inflammation within the brain or spinal cord, which can damage nerve cells and contribute to neurological diseases.
neurological: Relating to the brain and nervous system.
neurological disorder: A disease affecting the brain, spinal cord, or nervous system.
neuromuscular: Relating to both nerves and muscles, particularly the nerve signals that control muscle contraction.
neuromyelitis: Inflammation of both nerves and the spinal cord. Neuromyelitis optica is a rare autoimmune disease affecting the optic nerves and spinal cord.
neuromyelitis optica: A rare autoimmune disease where the immune system attacks the optic nerves and spinal cord, causing vision loss and paralysis.
neuromyelitis optica spectrum: A rare autoimmune disorder (NMOSD) where the immune system attacks the optic nerves and spinal cord, causing vision loss and paralysis.
neuronal ceroid lipofuscinosis: The medical name for Batten disease, a group of inherited disorders where toxic material (lipofuscin) builds up in brain cells.
neuronopathic: Affecting neurons (nerve cells) directly, causing cell death and neurological symptoms.
neuropathic: Caused by nerve damage. Neuropathic pain is a burning or shooting pain from damaged nerves.
neuropathic pain: Burning, tingling or shooting pain caused by damaged or misfiring nerves rather than by injury to tissue. In Fabry disease it often affects the hands and feet and can be set off by heat or exercise.
neuropathy: Nerve damage, often causing numbness, tingling, or pain in hands and feet.
neuroprotective: Capable of protecting nerve cells from damage or degeneration.
neuropsychiatric: Involving both neurological and psychiatric symptoms, such as mood changes, cognitive decline, or behavioral disturbances caused by brain disease.
neurotoxic: Capable of damaging nerve tissue. Some chemotherapy drugs cause neurotoxic side effects like numbness or tingling.
neurotransmitter: A chemical messenger that allows nerve cells to communicate with each other.
neurotrophic: Supporting the growth, survival, and maintenance of nerve cells. Neurotrophic factors are proteins that nourish neurons.
neurovisceral: Affecting both the nervous system and internal organs (viscera), as seen in certain porphyrias and metabolic diseases.
neutropenia: Abnormally low white blood cell count, which increases infection risk.
neutrophil: A type of white blood cell that fights bacterial infections.
neutrophil elastase: An enzyme white blood cells release to fight infection. Unchecked, it digests the elastic tissue of the lungs; alpha-1 antitrypsin normally keeps it in check.
newborn screening: A blood test performed on all newborns, usually by heel prick, to find serious but treatable conditions before symptoms appear. Panels include conditions such as phenylketonuria, sickle cell disease, methylmalonic acidemia and spinal muscular atrophy.
next-generation sequencing: Advanced genetic testing technology that reads millions of DNA fragments simultaneously to detect mutations.
next-generation TRK inhibitor: Newer TRK-blocking drugs designed to overcome resistance that develops to first-generation TRK inhibitors.
NF1 gene: The gene that provides instructions for making neurofibromin, a tumor suppressor. Mutations in this gene cause neurofibromatosis type 1.
NGS: Next-Generation Sequencing, a high-throughput method for reading DNA to detect cancer-driving mutations.
NHLBI: National Heart, Lung, and Blood Institute; a division of the NIH that publishes clinical guidelines for conditions including sickle cell disease.
night blindness: Difficulty seeing in low light or darkness, often the earliest symptom of retinitis pigmentosa as rod photoreceptors deteriorate.
NIMBLE: Regeneron's Phase 3 trial of cemdisiran in generalized myasthenia gravis, published in The Lancet in 2026. Cemdisiran improved daily-living scores by 2.3 points more than placebo at week 24.
nipocalimab: An FcRn-blocking antibody (brand Imaavy) that lowers harmful IgG antibodies, approved for generalized myasthenia gravis.
nivolumab: An immunotherapy antibody (brand Opdivo) that releases the immune system's brakes (PD-1) so it can attack cancer cells.
nizubaglustat: An investigational oral drug that reduces buildup of fatty molecules in brain cells, studied in GM1/GM2 gangliosidoses and Niemann-Pick type C.
NMD-670: The development code for ignaseclant, a ClC-1 inhibitor developed by NMD Pharma for Charcot-Marie-Tooth disease.
NMOSD: Neuromyelitis Optica Spectrum Disorder, a rare autoimmune condition attacking the optic nerves and spinal cord.
non-ambulatory: Unable to walk independently, typically using a wheelchair. Some trials have specific cohorts for non-ambulatory patients.
non-inferiority: A type of clinical trial designed to show a new treatment is not meaningfully worse than an existing treatment, rather than proving it is better.
non-randomized: A study design where the research team assigns patients to groups based on specific criteria (like age) rather than by random chance.
nondepolarizing: A type of muscle relaxant (rocuronium, vecuronium) that blocks nerve signals to muscles. Myasthenia gravis patients are extremely sensitive and can experience prolonged paralysis at standard doses.
nonpeptide: A drug built as a stable small molecule rather than a chain of amino acids. Peptide drugs are usually destroyed by digestion; nonpeptide drugs can survive the stomach, which is what makes a daily pill possible.
nonsense mutation: A change in DNA that creates a premature stop signal, producing a shortened, non-functional protein.
NORD: National Organization for Rare Disorders — a patient advocacy organization providing disease guides for rare conditions.
norepinephrine: A hormone and neurotransmitter (also called noradrenaline) that increases heart rate and blood pressure and plays a role in attention and alertness.
North Star Ambulatory Assessment: A 17-item rating scale (scored 0 to 34) used in Duchenne muscular dystrophy trials to measure a child's ability to stand, walk, climb, and perform motor tasks.
NSAA: North Star Ambulatory Assessment, a 17-item motor function scale used in Duchenne muscular dystrophy trials, scored from 0 to 34.
NT-proBNP: A blood marker released when the heart muscle is under strain. Higher levels mean more strain, and it is one of the 2 tests used to stage ATTR cardiomyopathy.
NTRK fusion: A genetic rearrangement joining an NTRK gene with another gene, creating an abnormal protein that drives cancer growth. Found across many cancer types.
NTRK1: One of the 3 NTRK genes. It carries the instructions for a nerve-growth receptor called TRKA. When NTRK1 fuses with another gene, the result can drive cancer growth, and TRK inhibitors such as Vitrakvi block it.
NTRK2: One of the 3 NTRK genes. It makes a nerve-growth receptor called TRKB. Cancers with an NTRK2 fusion are treated with the same TRK inhibitor drugs as the other types.
Nurtec: Brand name for rimegepant, a CGRP receptor antagonist approved for migraine treatment and prevention. Originally developed by Biohaven and acquired by Pfizer in 2022 for $11.6 billion.
nystagmus: Involuntary rapid, repetitive eye movements that can reduce vision and affect balance.

O

objective response rate: The percentage of patients whose tumors shrank or disappeared in response to treatment.
Oclaiz: Camurus's monthly octreotide depot that patients inject under the skin themselves (CAM2029), approved in Europe as Oczyesa and under FDA review with a December 18, 2026 decision date.
ocrelizumab: An antibody (brand Ocrevus) that depletes B cells, approved for relapsing and primary progressive multiple sclerosis.
Ocrevus Zunovo: A version of ocrelizumab combined with hyaluronidase so it can be given as an approximately 10-minute injection under the skin by a clinician twice a year, approved in September 2024.
octreotide: A synthetic version of the hormone somatostatin (brand Sandostatin) that suppresses growth hormone, used in acromegaly and neuroendocrine tumors.
oculomotor apraxia: Difficulty starting voluntary eye movements, so the eyes cannot quickly shift to look at something to the side; the head often turns first. A classic sign of ataxia-telangiectasia.
ofatumumab: A self-injected B cell-depleting antibody (brand Kesimpta) approved for relapsing multiple sclerosis.
off-label: Using an approved drug for a disease or patient group its FDA label does not list. It is legal and common in rare disease, but insurers can decline to pay for it.
OMIM: Online Mendelian Inheritance in Man — a catalog of genetic disorders maintained by Johns Hopkins University.
onasemnogene abeparvovec: A one-time gene therapy (brand name Zolgensma) for spinal muscular atrophy that delivers a functional copy of the SMN1 gene via an AAV9 vector.
oncogenic: Having the ability to cause normal cells to become cancerous. Oncogenic mutations drive tumor formation.
Onpattro: Brand name of patisiran, the first RNA interference drug ever approved (2018), given as an IV infusion every 3 weeks for hereditary ATTR polyneuropathy.
opakalim: A selective Kv7.2/7.3 potassium channel opener developed by Biohaven for focal epilepsy and inherited erythromelalgia. Unlike older Kv7 drugs, it avoids GABA receptor activity that caused safety problems. In Phase 2/3 testing for focal epilepsy, with RISE 3 results expected in the second half of 2026. Since September 4, 2026, an FDA partial clinical hold has paused new enrollment in its studies.
open-label: A trial where both researchers and patients know what treatment is being given.
ophthalmopathy: Disease affecting the eyes or the tissues around them, as in thyroid eye disease.
ophthalmoplegia: Paralysis of one or more of the muscles that control eye movement.
opportunistic infection: An infection from a germ that rarely harms people with a healthy immune system, which can take hold when the immune system is suppressed.
optic disc pallor: A pale appearance of the optic nerve head seen during an eye exam, indicating nerve fiber loss from retinal degeneration.
optic neuritis: Inflammation of the optic nerve that carries visual signals to the brain, causing pain with eye movement and temporary vision loss.
orbital decompression: Surgery that removes some of the bone or fat around the eye socket to make room for swollen tissue, allowing the eyeball to settle back.
orbital fibroblast: Connective tissue cells behind the eye that, in thyroid eye disease, are switched on by the immune system to multiply, make fat and swell, pushing the eye forward.
organomegaly: Abnormal enlargement of one or more organs, particularly the liver, spleen, or kidneys.
ORIGIN 3: Vera Therapeutics' Phase 3 trial of atacicept in adults with IgA nephropathy, with 428 patients in its final analysis. Its 36-week urine protein results supported the Trutakna approval, and its 2-year kidney function results were reported in September 2026.
orphan drug: A medication developed to treat a rare disease affecting fewer than 200,000 people in the U.S., eligible for special FDA incentives.
orphan drug designation: An FDA status for drugs targeting diseases affecting fewer than 200,000 Americans. It grants the developer incentives including 7 years of market exclusivity after approval, tax credits, and fee waivers.
Orphanet: A European information system on rare diseases and orphan drugs.
orthopnea: Difficulty breathing when lying flat, often a sign of heart failure. Patients may need to prop themselves up with pillows to sleep.
orthostatic hypotension: A sudden drop in blood pressure upon standing up, causing dizziness or fainting.
OspA: Outer surface protein A of Borrelia burgdorferi. A target for Lyme vaccines, including the discontinued LYMErix and the current candidate VLA15.
ossification: The process of bone formation, or the abnormal hardening of soft tissue into bone-like material.
osteoarthritis: The most common form of arthritis, caused by wear-and-tear breakdown of joint cartilage over time.
osteomalacia: Softening of the bones from impaired mineralization, often related to vitamin D or phosphate problems. Can occur in Wilson disease alongside osteopenia.
osteomyelitis: A bone infection, usually caused by bacteria, that can cause pain, swelling, and bone destruction.
osteopenia: Lower than normal bone density that is not severe enough to be classified as osteoporosis, but increases fracture risk.
osteoporosis: A condition where bones become weak and brittle due to loss of bone density, greatly increasing fracture risk.
otitis: Inflammation of the ear. Otitis media (middle ear infection) is especially common in children.
out-of-pocket maximum: The most a health plan member has to pay for covered care in a year. Once it is reached, the plan pays 100% of covered costs for the rest of that year.
ovine: From sheep.
Oxford MEST-C: A standardized scoring system used by pathologists to grade the severity of kidney damage in IgA nephropathy biopsy samples.
oxidative stress: An imbalance between harmful free radicals and the body's ability to neutralize them, causing cell damage linked to many diseases.

P

p-value: A number estimating how likely a gap at least this large would be if the treatment had no real effect. Below 0.05 is the usual bar for statistical significance.
pacritinib: A JAK2 inhibitor pill sold as Vonjo. The FDA approved it for myelofibrosis in people whose platelet count is below 50,000 per microliter, which is very low.
palliative: Focused on relieving symptoms and improving quality of life, not curing the disease.
palliative care: Specialized medical care focused on relieving symptoms and improving quality of life, regardless of the stage of disease.
palpitation: The sensation of your heart beating unusually hard, fast, or irregularly. Often described as a fluttering or pounding feeling in the chest.
paltusotine: A once-daily pill (brand name Palsonify) that activates the somatostatin receptor to lower growth hormone, approved in September 2025 as the first daily oral acromegaly treatment.
Palynziq: Brand name for pegvaliase, an enzyme substitution therapy approved for adults with phenylketonuria (PKU) whose blood phenylalanine levels are not adequately controlled by other treatments.
pancreatitis: Inflammation of the pancreas, causing severe abdominal pain. Can be acute (sudden) or chronic (long-lasting).
pancytopenia: A condition where red blood cells, white blood cells, and platelets are all lower than normal, weakening the body's ability to fight infection and clot blood.
panmyelopathy: Disorder affecting all cell lines produced by the bone marrow, reducing red cells, white cells, and platelets simultaneously.
papillary: Having a finger-like or nipple-like shape. Papillary thyroid cancer is the most common type of thyroid cancer.
papilloma: A benign, wart-like growth caused by HPV. In RRP, papillomas grow on the vocal cords and airway and come back after they are removed.
papillomatosis: Wart-like growths caused by HPV. In recurrent respiratory papillomatosis, they grow in the airway and keep coming back after surgery.
Papzimeos: Brand name of zopapogene imadenovec, Precigen's immunotherapy approved in August 2025 as the first treatment for adults with recurrent respiratory papillomatosis.
paradoxical neurologic worsening: A temporary worsening of neurologic symptoms (tremor, dystonia, dysarthria) during the first weeks of starting a copper chelator in Wilson disease, before symptoms improve as copper is removed.
parafollicular: Relating to specialized thyroid cells (C cells) that produce calcitonin. Medullary thyroid cancer arises from these cells.
parafollicular C-cell: Specialized thyroid cells that produce calcitonin. Medullary thyroid cancer arises from these cells, unlike common thyroid cancers.
parasitemia: The presence of parasites in the bloodstream. In babesiosis, measured by blood smear to determine infection severity.
parathyroid: Four small glands behind the thyroid that produce parathyroid hormone (PTH) to regulate calcium levels in the blood and bones.
paresthesia: Abnormal sensations like tingling, numbness, or pins-and-needles feeling.
parkinsonism: Symptoms resembling Parkinson's disease (tremor, stiffness, slow movement) that can be caused by various conditions or medications.
paroxysmal: Occurring in sudden, unpredictable episodes or attacks that start and stop abruptly.
partial response: The tumor has shrunk significantly (usually by at least 30%) but has not completely disappeared.
pasireotide: A somatostatin analog (brand Signifor) that suppresses hormone overproduction in Cushing disease and acromegaly.
pathogen: Any microorganism (bacterium, virus, parasite) capable of causing disease.
pathogenic: Capable of causing disease. A pathogenic variant is a genetic change confirmed to cause a medical condition.
pathogenic variant: A genetic change confirmed to cause disease. The modern term for what was previously called a disease-causing mutation.
pathognomonic: A sign or symptom that is highly characteristic and diagnostic of a specific disease.
pathologic fracture: A fracture that occurs spontaneously or with minimal trauma due to weakened bone.
pathologically confirmed: Confirmed by lab analysis of tissue or cells.
patient advocacy: Activities by organizations or individuals promoting patient rights, education, and access to treatments.
patient registry: A database that collects health information from people with a specific disease over time.
patient-reported outcome: Measurements of health status reported directly by patients rather than clinicians.
patisiran: An RNA interference TTR silencer (brand name Onpattro) infused into a vein every 3 weeks, with premedication, for hereditary ATTR polyneuropathy.
PCR: Polymerase Chain Reaction; a lab technique that amplifies small amounts of DNA to detect pathogens with high sensitivity.
PCSK1: A gene for an enzyme that processes hunger-regulating hormones. PCSK1 deficiency causes a rare genetic obesity disorder.
PDUFA: The Prescription Drug User Fee Act, the law under which the FDA commits to a decision deadline for each drug application. A drug's PDUFA date is the day by which the FDA has agreed to approve or reject it.
PDUFA date: The deadline by which the FDA has committed to decide on a drug application, set under the Prescription Drug User Fee Act. Often called the action date; the agency sometimes rules early or extends it.
pegcetacoplan: A complement C3 inhibitor (brand name Empaveli) approved for PNH and being studied in C3 glomerulopathy. Works by blocking C3 cleavage.
pegvaliase: An injectable enzyme substitution therapy (brand name Palynziq) that breaks down phenylalanine in the blood, used for adults with PKU who cannot control levels through diet or other drugs.
Pemazyre: Brand name for pemigatinib, the first targeted pill approved for bile duct cancer with an FGFR2 fusion (2020).
pembrolizumab: An immunotherapy antibody (brand Keytruda) that releases the immune system's PD-1 brake so it can recognize and attack cancer cells.
PEMF: Pulsed Electromagnetic Field therapy; low-energy electromagnetic pulses applied to tissues. Has peer-reviewed research showing effects on bacterial biofilms.
pemigatinib: An FGFR-blocking pill (brand Pemazyre) approved for bile duct cancer with FGFR2 fusions.
pemphigoid: A group of autoimmune skin diseases that cause large, tense blisters. The immune system attacks proteins anchoring the skin layers together.
pemphigus: A group of serious autoimmune skin diseases where the immune system attacks proteins holding skin cells together, causing fragile blisters and erosions.
pemvidutide: An investigational weekly injection combining GLP-1 and glucagon activity, studied in fatty liver disease (MASH) and obesity.
penetrance: The percentage of people with a disease gene who actually develop the disease.
per-protocol: An analysis that only includes patients who completed the study exactly as planned, without deviations.
performance status: A score doctors use to measure how well you can carry out everyday activities.
peripheral nervous system: The network of nerves outside the brain and spinal cord that carries signals to and from the limbs and organs.
peripheral neuropathy: Damage to nerves in the hands and feet causing numbness, tingling, pain, or weakness.
peripheral thyrotoxicosis: Too much thyroid hormone acting on organs outside the brain, such as the heart, muscle and liver. In MCT8 deficiency this happens because T3 that cannot get into the brain accumulates in the blood.
peritoneal: Relating to the peritoneum, the membrane lining the abdominal cavity and covering the abdominal organs.
peritubular capillary: Tiny blood vessels surrounding the kidney tubules. In Fabry disease, GL-3 accumulates here, and biopsies measure this to assess treatment response.
periventricular: Located around the brain's ventricles (fluid-filled spaces). White matter damage here is common in some neurological diseases.
peroxisomal: Relating to peroxisomes, small structures inside cells that break down fatty acids. Peroxisomal disorders cause toxic fat buildup.
pes cavus: Abnormally high arches in the foot, caused by chronic muscle imbalance. A characteristic feature of Charcot-Marie-Tooth disease.
PET scan: Positron Emission Tomography — an imaging test that shows cellular activity in the body.
petechiae: Tiny pinpoint red or purple spots on the skin from broken capillaries, often a sign of low platelet counts.
PF4: Platelet factor 4, a protein released by clotting platelets. Immune reactions against PF4 can trigger rare clotting disorders.
phagocytic: Relating to phagocytes, immune cells that engulf and destroy bacteria, dead cells, and other foreign material.
pharmacodynamic: How a drug affects the body, including its mechanism of action and the relationship between dose and effect.
pharmacokinetic: How a drug is absorbed, distributed, and eliminated by your body.
pharmacological chaperone: A small-molecule drug that helps misfolded enzymes fold correctly so they can reach their target cells and function. Migalastat (Galafold) for Fabry disease is an example.
pharyngeal: Relating to the pharynx (throat), the passage connecting the mouth and nose to the esophagus and windpipe.
Phase 3: The largest and last stage of testing before a company asks the FDA to approve a drug, usually comparing it against placebo in hundreds or thousands of patients.
phenomenon: An observable event or occurrence. In medicine, it refers to a specific clinical sign or pattern, such as Raynaud's phenomenon.
phenotype: The observable characteristics of an organism, which result from genes and environment.
phenylalanine: An essential amino acid found in protein-rich foods. People with PKU cannot break it down, causing toxic buildup.
pheochromocytoma: A rare tumor of the adrenal gland that causes excess adrenaline. Screened for in MEN 2 patients before thyroid surgery.
phlebotomy: Drawing blood from a vein, either for testing or as a treatment to reduce excess iron or red blood cells.
phospholipid: Fat molecules that form the basic structure of cell membranes, creating a flexible barrier around every cell.
photoreceptor: Light-sensing cells in the retina at the back of the eye. Their progressive death causes the vision loss in retinitis pigmentosa.
photosensitivity: Skin that burns or breaks out in a rash more easily in sunlight, a known side effect of pirfenidone and some other medicines.
physiological: Relating to the normal functions and processes of the body.
PI3K: Phosphoinositide 3-kinase, an enzyme that relays growth and survival signals inside cells. The delta form is important in immune cells; an overactive version causes APDS.
PIK3CA: A gene in a major cell-growth pathway. PIK3CA mutations are common in breast and endometrial cancers and are the target of newer precision drugs.
pili torti: A hair shaft abnormality where individual hairs are flattened and twisted at intervals. A hallmark finding in Menkes disease.
PIPE-791: Contineum Therapeutics' experimental LPA1 drug for IPF, a rival to admilparant with a different chemical structure and a lower dose, in Phase 2 testing.
pipeline: A company's collection of drugs in various stages of development, from early research through approval.
pirtobrutinib: A non-covalent (reversible) BTK inhibitor (brand name Jaypirca) approved for relapsed or refractory mantle cell lymphoma.
pituitary adenoma: A usually benign tumor growing in the pituitary gland at the base of the brain. The most common cause of acromegaly.
pituitary fossa: The small bony pocket at the base of the skull (also called the sella turcica) that holds the pituitary gland. A growth-hormone tumor can enlarge it.
pivotal trial: A definitive clinical trial whose results form the primary basis for evaluating a drug for FDA approval.
PKP2: A gene that makes a protein holding heart muscle cells together. PKP2 mutations cause ARVC, a rare inherited heart rhythm disease.
placebo: An inactive look-alike treatment (sugar pill or sham injection) given to the comparison group in a trial, so researchers can tell how much of the benefit comes from the drug itself.
placebo-adjusted: The drug's effect after accounting for what happened in the placebo group, which shows how much of the change came from the drug itself.
placebo-controlled: A study that compares the experimental treatment against an inactive substance (placebo) to measure the true effect.
plasma cell: White blood cells that live mostly in the bone marrow and make antibodies. Multiple myeloma is a cancer of these cells.
plasma exchange: A procedure where a patient's blood plasma is removed and replaced with donor plasma or a substitute, removing harmful antibodies.
plasmablast: Young antibody-making cells that B cells turn into. They carry CD19 on their surface, which is why anti-CD19 drugs like inebilizumab can remove them.
plasmapheresis: A procedure that removes, treats, and returns blood plasma. Used to clear harmful antibodies in autoimmune diseases.
plasmid: A small circular piece of DNA used in research and manufacturing to carry genetic instructions into cells.
platelet: Blood cells that help your blood clot. Low count means higher bleeding risk.
pleural: The double-layered membrane surrounding the lungs. Fluid can accumulate between the layers (pleural effusion) in some diseases.
pleurectomy: Surgical removal of part or all of the pleura (the membrane surrounding the lungs).
plexiform neurofibroma: A type of nerve sheath tumor that grows along the length of a nerve, often large and complex. Found in about 30–50% of people with NF1 and can cause pain, disfigurement, or compress nearby structures.
PMP22: Peripheral Myelin Protein 22, a gene whose extra copy causes CMT1A, the most common form of CMT. Investigational therapies aim to lower the body's PMP22 production back to normal.
pneumonectomy: Surgical removal of an entire lung, usually performed for lung cancer.
pneumonitis: Inflammation of lung tissue, often caused by an allergic reaction, infection, or as a side effect of certain drugs.
pneumothorax: A collapsed lung caused by air leaking into the space between the lung and chest wall.
polyangiitis: Inflammation affecting multiple blood vessels throughout the body, often as part of an autoimmune condition.
polyarteritis: Inflammation of medium-sized arteries that can restrict blood flow and damage organs throughout the body.
polycythemia: Too many red blood cells, thickening the blood. Polycythemia vera is a rare bone marrow disease that drives this overproduction.
polygenic: Involving multiple genes. Polygenic conditions are influenced by variations in many genes working together.
polyglutamine: An abnormally long chain of glutamine amino acids in a protein, caused by CAG repeat expansions.
polymyositis: An inflammatory disease causing weakness in the muscles closest to the trunk, making it difficult to climb stairs or lift objects.
polyneuropathy: Simultaneous damage to many nerves throughout the body, typically causing numbness and weakness starting in the hands and feet.
Pombiliti: Brand name for cipaglucosidase alfa, a next-generation enzyme replacement therapy for Pompe disease. Used in combination with Opfolda (miglustat) to improve enzyme uptake. Made by Amicus Therapeutics.
porcine: From pigs. On a drug label, a porcine-derived ingredient came from pig tissue.
post-marketing: Activities that occur after a drug is approved and on the market, including ongoing safety monitoring and additional studies.
postural instability: Loss of automatic reflexes to maintain balance and posture.
potassium channel opener: A drug that activates potassium channels on nerve cells, allowing potassium to flow out and reducing the cell's tendency to fire. Used to treat conditions caused by overactive nerves, like epilepsy.
povetacicept: Vertex's investigational drug that blocks BAFF and APRIL, injected under the skin every 4 weeks in trials. The FDA decision for IgA nephropathy is due November 30, 2026.
pralsetinib: A selective RET inhibitor (brand name Gavreto) approved for RET-altered thyroid cancers and RET fusion-positive lung cancer.
pre-medication: Medications given before a treatment (like an infusion) to prevent or reduce side effects such as allergic reactions.
preclinical: Laboratory and animal testing conducted before human trials to determine if a drug is safe enough to test in people.
predictive biomarker: A measurable indicator that helps predict whether a patient will respond to a specific treatment.
predisposition: An increased likelihood of developing a disease due to genetic or environmental factors.
premedication: Medicines given shortly before an infusion, usually a steroid and an antihistamine, to reduce the chance of an infusion reaction.
PREMIER trial: The Phase 3 trial of PXT3003 (Pharnext) in adults with CMT1A. Did not meet its primary endpoint.
prenatal diagnosis: Testing performed during pregnancy to determine whether an unborn baby has a genetic condition or birth defect.
presyncope: The feeling that you are about to faint, with symptoms like dizziness, lightheadedness, and blurred vision.
prevalence: How common a disease or condition is in a given population.
primary endpoint: The main result a clinical trial is designed to measure, such as tumor shrinkage or survival time.
principal investigator: The lead doctor or researcher responsible for running a clinical trial at a specific study site. Often abbreviated as PI.
prior authorization: A review where your insurer decides whether it will pay for a prescribed treatment before you receive it. Denials can be appealed, often successfully, with complete medical documentation.
priority review: An FDA designation meaning a drug application will be reviewed within 6 months instead of the standard 10-month period.
Priority Review Voucher: A transferable FDA voucher awarded to a company that wins approval of a drug for a rare pediatric disease or certain other conditions. It entitles the holder to a faster 6-month review of any future drug and can be sold to another company, typically for $100 million or more.
prognosis: The expected course and likely outcome of a disease.
prognostic: Relating to the predicted course or outcome of a disease. A prognostic marker helps estimate how a disease will behave.
progression-free survival: The length of time during and after treatment that a patient's disease does not get worse.
progressive disease: Disease that is actively getting worse.
progressive pulmonary fibrosis: Lung scarring from a disease other than IPF, such as rheumatoid arthritis or sarcoidosis, that keeps getting worse despite treatment.
prolactin: A hormone produced by the pituitary gland that stimulates milk production and has many other effects on metabolism and immune function.
promyostatin: The inactive precursor form of myostatin, before the body switches it on. Some drugs target this form rather than active myostatin to avoid affecting related proteins.
PROPEL trial: A Phase III clinical trial studying Pombiliti + Opfolda (cipaglucosidase alfa + miglustat) in late-onset Pompe disease patients already on enzyme replacement therapy.
prophylactic thyroidectomy: Preventive surgical removal of the thyroid gland before cancer develops. Done in childhood for people carrying hereditary RET mutations.
prophylaxis: A treatment or action taken to prevent a disease or condition before it occurs.
proptosis: Forward bulging of the eye out of its socket, the hallmark physical change of thyroid eye disease.
prospective study: A study that follows patients forward in time from the start of the research, collecting data as events happen.
proteasome inhibitor: Drugs such as bortezomib (Velcade) and carfilzomib (Kyprolis) that block the cell's protein-recycling machinery, which myeloma cells depend on more than normal cells do.
PROTECT trial: A Phase III clinical trial studying sparsentan in IgA nephropathy, measuring proteinuria reduction and kidney function preservation.
protein-to-creatinine ratio: A urine test (UPCR) that measures how much protein your kidneys are leaking. A key marker for tracking IgA nephropathy progression.
proteinosis: Abnormal protein buildup. In pulmonary alveolar proteinosis, protein-rich material accumulates in the lungs' air sacs and impairs breathing.
proteinuria: Excess protein in the urine, a sign that the kidneys are not filtering properly. A key marker in IgA nephropathy and Fabry disease.
protocol: The detailed plan for how a clinical trial will be conducted.
proton pump inhibitor: A class of acid-reducing stomach medications (omeprazole, esomeprazole, pantoprazole and others) used for reflux and ulcers. They can change how much of certain other drugs the body absorbs.
proximal: Relating to the part of a limb closest to the body center.
pruritus: Medical term for itching, which can be caused by skin conditions, allergies, liver disease, or other internal disorders.
pseudobulbar: Relating to conditions that mimic damage to the brainstem's bulbar region, causing difficulty with speech, swallowing, or involuntary laughing/crying.
psoriasis: A chronic autoimmune skin disease that speeds up skin cell growth, causing thick, scaly patches on the skin surface.
psychosis: A mental state where a person has difficulty distinguishing what is real from what is not, sometimes involving hallucinations or delusions.
ptosis: Drooping of the eyelid due to muscle weakness.
PubMed: The National Library of Medicine's free database of biomedical literature citations and abstracts.
pulmonary: Relating to the lungs.
pulmonary arterial hypertension: A rare, progressive disease in which the small arteries of the lungs narrow and stiffen, raising blood pressure in the lungs and straining the right side of the heart.
pulmonary embolism: A blood clot that blocks an artery in the lungs, causing chest pain, shortness of breath, and potentially death.
pulmonary function test: Breathing tests that measure how well your lungs are working, including how much air you can inhale and exhale and how efficiently oxygen enters your blood.
Purkinje cell: Large nerve cells in the cerebellum that coordinate movement. They are the cells lost in ataxia-telangiectasia and many other ataxias.
purpura: Purple or red spots on the skin caused by bleeding underneath, resulting from broken small blood vessels.
pustule: Small raised bumps filled with pus. In generalized pustular psoriasis the pus is made of white blood cells, not bacteria, so the pustules are not infected and not contagious.
PXT3003: A fixed-dose combination of baclofen, sorbitol, and naltrexone tested for CMT1A. Originally developed by Pharnext and now associated with the Tianshili Group. Did not meet its primary endpoint in the Phase 3 PREMIER trial in adults.
pyelonephritis: A kidney infection, usually caused by bacteria traveling up from the bladder, causing fever and back pain.
pyridostigmine: An older pill (brand Mestinon) that slows the breakdown of acetylcholine, the chemical nerves use to signal muscles, so more of it reaches the muscle. It eases myasthenia gravis symptoms but does not change the immune attack behind them.
pyridoxine: Vitamin B6. Often co-administered with penicillamine in Wilson disease because penicillamine can deplete B6 in patients with marginal nutritional status.
pyrophosphate scan: A nuclear medicine scan (technetium pyrophosphate, or PYP) that lights up transthyretin amyloid in the heart, allowing ATTR cardiomyopathy to be diagnosed without a biopsy.
Pyrukynd: Brand name of mitapivat for pyruvate kinase deficiency, approved by the FDA in February 2022.
pyruvate dehydrogenase: An enzyme that feeds sugar breakdown products into the mitochondria's energy pathway. Its deficiency, usually from the X-linked PDHA1 gene, is one cause of Leigh syndrome.
pyruvate kinase: An enzyme red blood cells use to turn sugar into energy. Drugs that switch it on, like mitapivat and etavopivat, aim to help red cells last longer.

Q

QTc: A measurement of heart rhythm timing on an ECG. Prolonged QTc can indicate heart rhythm risk.
quadruple-blind: A trial where the patient, the treating doctor, the data analyst, and the outcome assessor all do not know which treatment each patient received.
qualified treatment center: A hospital that a gene therapy's maker has trained and certified to give that therapy. Most gene therapies are only available at a small number of these centers.
quality of life: A measure of overall well-being, including physical health, emotional state, and ability to do daily activities.
quiescent: A period when a disease is inactive or dormant, with no symptoms or progression.

R

RAAS: Renin-Angiotensin-Aldosterone System, a hormone system that regulates blood pressure. Blocking it is a cornerstone of kidney disease treatment.
RAAS blockade: Using medications (like ACE inhibitors or ARBs) to block the renin-angiotensin system, which helps control blood pressure and reduce kidney damage.
radiation therapy: Treatment using high-energy beams to destroy cancer cells.
radioactive iodine: A standard thyroid cancer treatment, taken as a capsule, in which thyroid cells absorb radioactive iodine and are destroyed from inside. Tumors that stop absorbing it are called RAI-refractory, which is when genomic testing for NTRK and other fusions is recommended.
radiopharmaceutical: A drug containing a radioactive substance, used either to image tumors on a scan or to deliver targeted radiation directly to cancer cells.
radiotherapy: Treatment using high-energy beams (same as radiation therapy).
RAI-refractory: Thyroid cancer that no longer takes up radioactive iodine, so that treatment stops working. Guidelines recommend testing these tumors for NTRK, RET, ALK and BRAF alterations.
RAINIER: Vertex's Phase 3 trial of povetacicept in 605 adults with IgA nephropathy. A planned 36-week analysis supports its FDA application, and a 2-year analysis will measure kidney function.
randomized: Assigned to treatment groups by chance, like a coin flip, so the groups start out comparable and differences in outcomes can be credited to the treatment.
randomized controlled trial: A study where patients are randomly assigned to receive either the treatment being tested or a comparison (like placebo), reducing bias.
randomized withdrawal: A trial design in which patients already stable on a drug are randomly assigned to keep taking it or to switch to placebo, to show that the drug is doing something by watching what happens when it is taken away.
RAPID trial: The largest placebo-controlled trial of AATD augmentation therapy (Lancet, 2015). Weekly Zemaira slowed loss of lung density measured at total lung capacity, though not on its other primary measures.
rapid-sequence intubation: An emergency procedure to quickly place a breathing tube using a sedative and muscle relaxant, used when a patient cannot breathe on their own.
rare pediatric disease designation: An FDA designation for a drug treating a serious rare disease that mainly affects children. On approval, the company becomes eligible for a Priority Review Voucher it can sell.
RAS inhibitor: Blood pressure medicines, usually an ACE inhibitor or an ARB, that also lower the pressure inside the kidney's filters and reduce protein leaking into urine. These are standard background care in IgA nephropathy, prescribed before and alongside newer targeted drugs.
RAS/MAPK: A cell signaling pathway that controls growth and division. Overactivation of this pathway drives tumor formation in NF1 and related conditions.
ravulizumab: A long-acting complement C5 blocker (brand Ultomiris), an every-8-weeks successor to eculizumab for PNH, aHUS, myasthenia gravis, and NMOSD.
reading frame: The way a cell reads a gene's letters in groups of 3. A deletion that keeps the groups lined up still makes a shortened protein (Becker muscular dystrophy); one that breaks them stops protein production a few steps later (Duchenne).
real-world evidence: Clinical data gathered from actual patient care settings outside of controlled clinical trials.
rebisufligene etisparvovec-hopf: The generic name for Fayuvi, a one-time gene therapy that delivers a working SGSH gene so cells can make the enzyme missing in Sanfilippo syndrome type A.
Reblozyl: Brand name for luspatercept, a treatment for anemia in adults with beta thalassemia who require regular red blood cell transfusions.
recessive: A pattern of inheritance where a condition appears only when both copies of a gene, one from each parent, carry the change. Parents with one copy each are usually unaffected carriers.
recurrence: The return of cancer after treatment and a period when no cancer could be detected.
recurrent: Disease that has come back after treatment.
recurrent laryngeal nerve: A nerve near the thyroid that controls the vocal cords. It can be damaged during thyroid surgery, causing voice changes.
redetermination: The first level of a Medicare Part D appeal, decided by the plan itself within 7 days (72 hours if expedited). You have 65 days from the denial notice to request it.
refractory: Disease that is not responding to current treatment.
refractory period: A time window after an event (like a seizure or heartbeat) during which the tissue cannot respond to a new stimulus.
registry: A database that collects information about patients with a particular disease.
regurgitation: Backward leaking of blood through a heart valve that does not close properly, forcing the heart to work harder.
REIGNITE: Agios's confirmatory Phase 3 trial of mitapivat in sickle cell disease, testing whether it keeps people aged 12 and older transfusion-free.
relapse: The return of disease signs and symptoms after a period of improvement or remission.
relapsed: Disease that came back after a period of improvement.
remission: A period when signs and symptoms of a disease are reduced or disappear.
REMS: Risk Evaluation and Mitigation Strategy: an FDA-required safety program for certain drugs, which can include extra monitoring, certified pharmacies, or prescriber training.
remyelination: The process of regaining or restoring myelin insulation around nerve fibers.
renal: Relating to the kidneys.
repotrectinib: A next-generation TRK inhibitor (brand name Augtyro) designed to work against cancers that have become resistant to earlier TRK inhibitors.
resection: Surgical removal of part or all of a tissue, organ, or tumor.
resection subgroup: A subset of study patients who had surgical removal of their tumor, analyzed separately to see how they responded to treatment.
respiratory chain: The series of 5 protein complexes inside mitochondria that turn food and oxygen into usable energy. Faults in complex I or complex IV are the commonest causes of Leigh syndrome.
respiratory failure: When the lungs can no longer adequately supply oxygen or remove carbon dioxide, requiring breathing support.
response assessment: A scheduled evaluation by your doctor to determine whether your treatment is working, based on lab results, imaging, symptoms, and functional tests.
resting tremor: Trembling that occurs when muscles are at rest, typical of Parkinson disease.
RET fusion: A chromosomal rearrangement joining the RET gene with a partner gene, creating an overactive protein that drives cancer in lung, thyroid, and other tissues.
RET fusion-positive: A cancer confirmed by testing to contain an RET gene fusion, making it potentially responsive to RET-targeted therapies.
RET inhibitor: A targeted drug that blocks the RET protein, stopping cancer growth driven by RET mutations or fusions.
RET mutation: A change in the RET gene that causes uncontrolled cell growth. Inherited RET mutations cause familial medullary thyroid cancer.
RET proto-oncogene: A gene encoding a growth-signaling protein. Mutations in RET drive medullary thyroid cancer and are used to predict cancer risk in families.
RET rearrangement: A chromosomal change that fuses the RET gene with another gene, producing an abnormal growth-driving protein found in some cancers.
reticulocyte: Young red blood cells recently released from the bone marrow. Their count indicates how fast the body is producing new red blood cells.
retinal: The light-sensitive tissue at the back of the eye that converts images into nerve signals sent to the brain.
retinal degeneration: Progressive damage to the light-sensing tissue at the back of the eye.
retinal dystrophy: Progressive degeneration of the retina (the light-sensing tissue at the back of the eye), leading to vision loss over time.
retinitis pigmentosa: A group of inherited eye diseases that cause progressive degeneration of the retina's light-sensing cells, leading to night blindness, tunnel vision, and eventual vision loss.
retinoid: A vitamin A-related class of drugs. In FOP, the retinoid palovarotene (Sohonos) dampens the downstream signal that drives abnormal bone growth.
retinopathy: Disease of the retina in the eye that can cause vision loss.
retroperitoneum: The space behind the abdominal cavity containing the kidneys, aorta, and other structures.
retrospective study: A study that looks back at data already collected (such as medical records) rather than following patients forward in time.
rhinitis: Inflammation of the lining of the nose, causing congestion, runny nose, and sneezing. Can be allergic or non-allergic.
ribitol: A sugar the body makes naturally. BBP-418 is an oral form given in large amounts so the partly working FKRP enzyme in LGMD2I has more raw material to finish its job.
risdiplam: An oral SMN2 splicing modifier (brand name Evrysdi) for spinal muscular atrophy that increases production of functional SMN protein.
RISE UP: Agios's Phase 2/3 trial of mitapivat in people 16 and older with sickle cell disease.
rituximab: A B cell-depleting antibody (brand Rituxan) used in blood cancers and many autoimmune diseases.
RNA interference: A biological mechanism that silences specific genes by blocking gene expression.
RNA-based sequencing: A test that reads the RNA copies a cell makes of its genes rather than the DNA itself. Because the copies have the large non-coding gaps already cut out, it catches gene fusions, such as NTRK3 fusions, that a DNA-only panel can miss.
Roctavian: Brand name for valoctocogene roxaparvovec, a gene therapy approved in 2023 for severe hemophilia A and withdrawn from the market by BioMarin in February 2026. Delivers a functional copy of the Factor VIII gene using an AAV5 viral vector.
ROS1: A gene that can fuse with another gene and drive cancer, most often a small share of lung cancers. Several TRK drugs, including Rozlytrek and Augtyro, also block ROS1.
rozanolixizumab: An FcRn-blocking antibody (brand Rystiggo) that lowers harmful IgG antibodies, approved for generalized myasthenia gravis.
Rozlytrek: Brand name for entrectinib, an FDA-approved therapy for NTRK fusion cancers that can also treat brain metastases.
RP: Retinitis Pigmentosa, a group of inherited retinal diseases causing progressive photoreceptor death and vision loss.
RPE65: A gene essential for the visual cycle in the retina. Mutations in RPE65 cause a treatable form of inherited retinal disease (Luxturna gene therapy).
rusfertide: A hepcidin mimetic (brand Mimrylo) that restrains iron use by the bone marrow, approved in 2026 for polycythemia vera.
ruxolitinib: A JAK1/JAK2 inhibitor (brand names Jakafi and Jakavi) that works by blocking the JAK-STAT signaling pathway. Approved for myelofibrosis, polycythemia vera and graft-versus-host disease.
Rydapt: Brand name for midostaurin, approved for advanced systemic mastocytosis and AML with FLT3 mutations.

S

SAFARI44: Novartis's Phase 3 confirmatory trial of del-zota in exon 44 amenable Duchenne, run outside the United States, with a saline placebo for 54 weeks followed by open-label drug.
salivary gland biopsy: A procedure to remove a small sample of salivary gland tissue (usually from the lip) to check for immune cell infiltration — a key diagnostic test for Sjögren's syndrome.
salvage therapy: Treatment tried after multiple previous treatments have failed.
SARA: Scale for the Assessment and Rating of Ataxia, a 40-point clinician-scored test of gait, stance, sitting, speech and limb coordination. Lower scores mean less ataxia.
sarcoma: A cancer that develops in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissues.
sarcomere: The basic contracting unit inside a muscle cell. Most inherited HCM is caused by mutations in genes that build the sarcomere.
sarcopenia: The gradual loss of muscle mass and strength that comes with aging.
sBLA: Supplemental Biologics License Application: a request to expand an already-approved biologic drug to a new use, dose, or patient group.
SCA2: Spinocerebellar ataxia type 2, an inherited disorder causing progressive loss of coordination, caused by a repeat expansion in the ATXN2 gene.
SCA3: Spinocerebellar ataxia type 3 (Machado-Joseph disease), the most common inherited ataxia, causing progressive loss of coordination.
SCA6: Spinocerebellar ataxia type 6, a slowly progressive inherited ataxia usually starting in mid-adult life.
SCA7: Spinocerebellar ataxia type 7, an inherited ataxia that also causes progressive vision loss from retinal degeneration.
SCAN-MP: A US study that screened 646 Black and Caribbean Hispanic adults over 60 with heart failure for ATTR heart disease. It found the disease in 6.7%, and in 17% of Black men over 75.
Scheie syndrome: The mildest form of MPS I with later onset and slower progression. Patients typically have normal intelligence but develop joint and organ problems.
scintigraphy: An imaging technique where a small amount of radioactive material is injected and a special camera creates pictures showing how organs function.
scleroderma: An autoimmune disease that causes hardening and tightening of the skin and connective tissues, and can affect internal organs.
sclerodermatous: Skin that has become thickened, tight, and hardened — a feature of chronic graft-versus-host disease that resembles scleroderma.
sclerosis: Hardening and scarring of tissue, often in the nervous system.
SCN1A: A gene for a sodium channel that calms electrical activity in the brain. SCN1A mutations cause Dravet syndrome and related epilepsies.
SCN2A: A gene for a sodium channel that brain cells use to fire signals. SCN2A mutations cause rare developmental epilepsies.
SCN8A: A gene for a sodium channel that brain cells use to fire signals. SCN8A mutations cause rare developmental epilepsies.
scoliosis: An abnormal sideways curvature of the spine, which can range from mild to severe.
scotoma: A blind spot or area of reduced vision in the visual field, often caused by damage to the retina or optic nerve.
screening: Testing done to detect a disease or condition early, often before symptoms appear.
second-line: Treatment given after the first treatment didn't work well enough.
secondary endpoint: An additional result measured in a clinical trial beyond the main goal, providing extra information about the treatment.
secretor: Someone whose blood group molecules appear in body fluids like saliva and plasma, not only on their blood cells. Most people are secretors, which is why a unit of type B plasma carries B antigen even with almost no red cells in it.
secretory breast: A rare subtype of breast cancer that commonly carries NTRK gene fusions and may respond to TRK inhibitor therapy.
secretory carcinoma: A rare, usually slow-growing cancer of the salivary gland (or breast) defined by an ETV6-NTRK3 gene fusion, which is present in 8 or 9 of every 10 cases. Known until 2017 as mammary analogue secretory carcinoma, or MASC.
selective RET inhibitor: A drug designed to precisely block RET while sparing other targets, resulting in fewer side effects than older multi-kinase drugs.
selitrectinib: An experimental next-generation TRK inhibitor, also known as LOXO-195, designed for tumors that stop responding to Vitrakvi. It has had no new trials since its Phase 1 study ended in January 2023.
selpercatinib: A selective RET inhibitor (brand name Retevmo) approved for RET-mutant medullary thyroid cancer and RET fusion cancers.
selumetinib: An oral MEK inhibitor (brand name Koselugo) FDA-approved for children with NF1-related inoperable plexiform neurofibromas.
Sensoready pen: The prefilled, single-use autoinjector pen Novartis uses for Kesimpta and some other drugs, designed for self-injection at home.
septal myectomy: Open-heart surgery that shaves away part of the thickened wall (septum) between the heart's chambers to relieve outflow obstruction.
sequelae: Lasting health effects that remain after a disease or treatment.
serious adverse event: A side effect that is life-threatening, requires hospitalization, or causes lasting harm.
serologic: Relating to blood tests that detect antibodies or antigens to diagnose infections or immune responses.
serology: Laboratory testing of blood serum for antibodies, used to work out whether the body has met a specific infection or is producing antibodies against its own tissue.
seronegative: Testing negative for specific antibodies in the blood, despite having symptoms of a disease.
seropositive: Testing positive for specific antibodies in the blood.
serotonin: A neurotransmitter involved in mood, sleep, and many body functions.
SERPINA1: The gene for alpha-1 antitrypsin, a protein that protects the lungs from damage. Mutations cause alpha-1 antitrypsin deficiency.
serum: The clear liquid part of blood that remains after blood cells and clotting factors have been removed. Many lab tests measure substances in serum.
serum calcitonin: A blood test measuring calcitonin levels. The primary marker for diagnosing and monitoring medullary thyroid cancer.
serum creatinine: A blood test measuring creatinine levels to assess kidney function.
setmelanotide: An MC4R-activating drug (brand Imcivree) that restores hunger signaling in rare genetic obesity disorders.
setrusumab: An experimental antibody from Ultragenyx and Mereo BioPharma that blocks sclerostin, a protein that slows bone building. It was tested to reduce fractures in osteogenesis imperfecta.
SGLT2 inhibitor: A type of medication originally developed for diabetes that also protects the kidneys by reducing pressure on the filtering units.
SGSH: The gene that makes sulfamidase, the enzyme that breaks down heparan sulfate. A fault in SGSH causes Sanfilippo syndrome type A.
sibeprenlimab: An antibody that blocks APRIL, a signal driving harmful IgA production, developed for IgA nephropathy.
sicca: Dryness of the eyes and mouth — the hallmark symptom complex in Sjögren's syndrome, caused by immune-mediated destruction of moisture-producing glands.
sIgE: Specific IgE, a blood test that measures antibodies to a particular allergen. The alpha-gal sIgE test is the standard diagnostic for Alpha-Gal Syndrome.
siltuximab: An antibody sold as Sylvant that blocks interleukin-6, an immune signal that drives inflammation. It is FDA approved for multicentric Castleman disease in people without HIV or HHV-8 infection.
single-arm: A trial in which every participant receives the treatment and there is no placebo or comparison group. It can show that a measurement changed, but not by itself prove the treatment caused the change.
single-blind: A trial where either the patient or the researcher (but not both) does not know which treatment is being given.
single-fiber EMG: A sensitive nerve and muscle test that records individual muscle fibers to spot the signal delays typical of myasthenia gravis. It often confirms the diagnosis when antibody blood tests come back negative.
sinusitis: Inflammation of the sinuses (air-filled spaces in the skull), causing congestion, facial pain, and thick nasal discharge.
siRNA: Small interfering RNA — a type of RNA molecule used to block specific genes from producing proteins.
sJIA: Systemic Juvenile Idiopathic Arthritis — a severe autoinflammatory form of childhood arthritis with daily spiking fevers, rash, and joint inflammation.
skeletal abnormality: Abnormal bone or skeletal structure and development.
skeletal dysplasia: A large group of genetic conditions that affect how bone and cartilage grow. Achondroplasia is the most common one.
Skytrofa: Brand name for lonapegsomatropin, a once-weekly growth hormone therapy by Ascendis Pharma for pediatric growth hormone deficiency.
SLC16A2: The gene that carries the instructions for the MCT8 transporter. Changes in SLC16A2 cause MCT8 deficiency. It sits on the X chromosome, which is why the disease affects boys almost exclusively.
slit-lamp: An eye exam tool that uses a focused beam of light and a microscope to inspect the front of the eye. Used to detect Kayser-Fleischer rings in Wilson disease.
small interfering RNA: A short piece of RNA that tells cells to make less of a specific protein. Qfitlia uses one to lower antithrombin in the liver.
small molecule: A type of drug small enough to cross cell membranes and reach intracellular targets.
SMN protein: Survival motor neuron protein. Motor neurons need it to stay alive, and spinal muscular atrophy happens when the body cannot make enough of it.
SMN1: The gene that normally makes SMN protein, which motor neurons need to survive. A fault in both copies of SMN1 is what causes spinal muscular atrophy.
SMN2-targeted: Describes drugs that work by pushing the backup gene SMN2 to make more of the missing SMN protein, which protects motor neurons. Spinraza and Evrysdi work this way.
SOD1: A gene whose mutations cause an inherited form of ALS. The approved drug tofersen works by silencing mutant SOD1.
sodium starch glycolate: A starch-based ingredient that helps tablets break apart quickly. Plant-derived.
soft tissue mineralization: Calcium and phosphate settling into soft tissues such as blood vessels, skin or the eye, a risk when blood phosphate stays high on FGFR drugs.
softgel: A capsule with a soft, one-piece shell, almost always made of gelatin, holding a liquid or oil. Common for fish oil, vitamin D, and liquid pain relievers.
solvent-front mutation: A change at the entrance of a protein's drug-binding pocket that physically blocks a drug from fitting. In NTRK fusion cancer these mutations are the usual reason Vitrakvi or Rozlytrek stops working, and Augtyro was designed to bind despite them.
somatic: Relating to body cells (not inherited). A somatic mutation occurs only in tumor cells and is not passed to children.
somatic mutation: A genetic change that occurs only in tumor cells, not inherited and not present from birth.
somatostatin analogue: A medication that mimics somatostatin, a natural hormone that inhibits growth hormone release. The main drug treatment for acromegaly.
Somavert: Brand name of pegvisomant, a daily injection that blocks the growth hormone receptor so growth hormone cannot raise IGF-1.
sotatercept: An activin signaling inhibitor (brand name Winrevair) approved in 2024 for pulmonary arterial hypertension (PAH). It works by rebalancing growth signals in blood vessel walls.
spasticity: Stiffness and increased muscle tension that can make movement difficult.
specialty pharmacy: A pharmacy that handles complex or expensive drugs a retail pharmacy cannot stock. Most new rare disease drugs ship from 1 or 2 contracted specialty pharmacies directly to your home.
spectrum disorder: A condition that presents across a range of severity and symptoms rather than as a single fixed set of features.
sphingolipidosis: A class of fats found in cell membranes. Sphingolipidoses are storage diseases where these fats accumulate due to enzyme deficiencies.
sphingomyelin: A type of fat found in cell membranes, particularly in nerve cell insulation. Builds up in Niemann-Pick disease.
spinal stenosis: Narrowing of the spinal canal that pinches nerves and causes back and leg pain. Transthyretin amyloid can build up in spinal ligaments, so it is one of the early warning signs of ATTR amyloidosis.
spinal tap: A procedure (lumbar puncture) where fluid is drawn from around the spinal cord through a needle for diagnostic testing.
spirochete: A type of corkscrew-shaped bacterium. Borrelia burgdorferi, the cause of Lyme disease, is a spirochete.
spirometry: A breathing test that measures how much air your lungs can hold and how quickly you can breathe it out.
splenectomy: Surgical removal of the spleen, sometimes needed for blood disorders or trauma. Increases risk of certain infections afterward.
splenic: Relating to the spleen, an organ that filters blood, recycles old red blood cells, and plays a role in immune defense.
splenomegaly: Enlargement of the spleen beyond normal size.
splicing: The process of cutting and reconnecting segments of RNA to produce the final message that directs protein production. Errors can cause disease.
sporadic: Occurring by chance without a family history of the disease.
sputum: Mucus and other material coughed up from the lungs and airways, often examined to diagnose respiratory infections.
stable disease: Disease that is not growing or shrinking.
stable-dose: Taking the same amount of a medicine on the same schedule for a set period. Many trials let you keep a medicine as long as the dose has not changed for a certain number of weeks before you enroll.
staging: Determining the extent and spread of cancer in the body, which guides treatment decisions. Stages range from I (localized) to IV (widespread).
standard of care: The accepted medical treatment approach for a specific disease based on evidence and clinical guidelines.
State Health Insurance Assistance Program: Free, federally funded counseling in every state that helps people with Medicare compare plans, understand costs and handle appeals. Find your state's program at shiphelp.org.
statistical significance: A result unlikely to have occurred by chance alone, typically meaning there is less than a 5% probability it is a fluke.
status epilepticus: A medical emergency where seizures occur continuously or in rapid succession without recovery.
steady state: The point where a medication has reached stable, consistent levels in the body after repeated doses — usually takes several weeks of regular dosing.
steatohepatitis: Liver inflammation driven by fat buildup in liver cells, which can scar the liver over time. Includes the condition known as MASH.
steatorrhea: Fatty, foul-smelling stools caused by the body's inability to properly digest and absorb fats.
steatosis: Abnormal buildup of fat inside liver cells (fatty liver), which can progress to inflammation and scarring.
stem cell therapy: Treatment using stem cells that can develop into different cell types to repair or replace damaged tissues and organs.
stem cell transplant: A procedure that replaces damaged or diseased bone marrow with healthy stem cells, which can then produce new blood cells.
step therapy: An insurance policy requiring patients to try cheaper or older medications first before the insurer will cover a newer or more expensive drug. Also called 'fail first,' because the patient must fail on the initial treatment before moving on. Many states have passed step therapy reform laws giving doctors the ability to override these requirements.
stereotactic biopsy: A biopsy in which a needle is guided to a precise spot in the brain using a frame or computer imaging, so a small tissue sample can be taken without open surgery.
steroidogenic: Relating to the production of steroid hormones (like cortisol and sex hormones) by the adrenal glands and reproductive organs.
stomatitis: Inflammation and sores inside the mouth, which can be caused by infections, medications, or autoimmune conditions.
storiform fibrosis: A distinctive pattern of tissue scarring with a swirling, cartwheel-like arrangement of collagen fibers. A hallmark histologic finding in IgG4-related disease.
stork-leg: An appearance where the calves look very thin while the thighs remain relatively normal-sized, caused by muscle wasting in the lower legs. A characteristic feature of Charcot-Marie-Tooth disease.
strabismus: A condition where the eyes do not align properly — one eye may turn in, out, up, or down while the other looks straight ahead.
stride velocity: A measure of walking speed captured by a wearable device, reflecting real-world mobility rather than clinic-based tests.
stridor: A high-pitched wheezing sound caused by turbulent airflow through a narrowed or obstructed airway.
subclinical: A disease or condition present in the body but not yet causing noticeable symptoms or detectable by standard examination.
subcortical: Relating to brain structures located beneath the cerebral cortex, involved in movement, emotion, and basic functions.
subcutaneous: Injected just under the skin.
subluxation: A partial dislocation of a joint, where the bones are misaligned but still partially in contact.
substantial evidence: The legal standard the FDA must meet to approve a drug for effectiveness, set in 1962. It has usually meant 2 adequate and well-controlled trials, but since 1997 can be met by 1 trial plus confirmatory evidence.
substrate: The substance that an enzyme acts upon. In metabolic diseases, substrates build up when the enzyme that processes them is missing.
substrate reduction: A treatment approach that lowers production of the substance building up in the body, rather than replacing the missing enzyme that would normally break it down.
succinylcholine: A depolarizing muscle relaxant used for rapid-sequence intubation. In myasthenia gravis, patients may be resistant and require higher doses.
SUDEP: Sudden unexpected death in epilepsy, a rare but real risk for people with frequent or poorly controlled seizures. Good seizure control and night-time monitoring lower the risk.
sulfamidase: The enzyme that breaks down heparan sulfate. It is missing or deficient in Sanfilippo syndrome type A because of a faulty SGSH gene.
sunitinib: A targeted cancer pill (brand Sutent) that blocks several growth signals including KIT, long used as the standard second-line treatment for GIST.
superiority: A clinical trial result showing one treatment is statistically better than another on a specific endpoint.
supportive care: Care focused on relieving symptoms and maintaining comfort rather than curing disease.
surfactant: The oily film that coats the inside of the lungs' air sacs and keeps them from collapsing. In pulmonary alveolar proteinosis it builds up instead of being cleared away.
surrogate endpoint: A measurement used as a stand-in for the outcome that actually matters, chosen because it shows up years sooner. Lowering protein in the urine stands in for keeping a patient off dialysis.
sutimlimab: An antibody (brand Enjaymo) that blocks the C1s complement protein, approved for cold agglutinin disease.
SV2A: A protein on nerve-signal vesicles in the brain, the target of the seizure medicines levetiracetam and brivaracetam.
switch therapy: Changing from one medication to another, typically because the first treatment was not working well enough or causing side effects.
symptomatic: Showing noticeable symptoms of a condition.
synapse: The connection point between two nerve cells where chemical signals are transmitted.
SYNAPSE-CMT trial: The Phase 2a randomized, double-blind, placebo-controlled trial (NCT06482437) of ignaseclant (NMD670) in 81 patients with CMT Type 1 and Type 2.
syncope: Fainting; a sudden loss of consciousness due to decreased blood flow to the brain.
systemic: Affecting the entire body rather than a single organ or area.
systemic juvenile idiopathic arthritis: A severe form of childhood arthritis characterized by spiking daily fevers, salmon-colored rash, joint inflammation, and elevated inflammatory markers. Driven by the innate immune system rather than typical autoimmunity.
systemic mastocytosis: A rare disease where abnormal mast cells accumulate in organs like bone marrow, liver, and spleen, causing a wide range of symptoms from skin rashes to life-threatening reactions.
systemic therapy: Treatment that travels through the bloodstream to reach cells throughout the body.
systolic: Relating to the phase when the heart muscle contracts and pumps blood out. Systolic blood pressure is the top number in a reading.

T

T-cell: A type of white blood cell that directly attacks infected or abnormal cells and coordinates the immune response.
T119M: A variant in the TTR gene that makes transthyretin unusually stable. People who carry it alongside a disease-causing variant are often protected, which inspired the design of acoramidis.
T3: Triiodothyronine, the active form of thyroid hormone. It sets the body's metabolic rate and is essential for brain development. Blood T3 is high in MCT8 deficiency because it cannot enter the brain.
T4: Thyroxine, the main hormone the thyroid gland releases, which the body converts into the active hormone T3. Blood T4 is typically low in MCT8 deficiency.
tachycardia: An abnormally fast heart rate.
tachyphylaxis: A rapid decrease in a drug's effectiveness after repeated doses, requiring higher doses to achieve the same effect.
tachypnea: Abnormally rapid breathing, often a sign that the body is not getting enough oxygen.
TACI: A receptor on B cells that catches the BAFF and APRIL signals. Atacicept and povetacicept are built from its outer piece, which lets them soak up both signals before they reach B cells.
tacrolimus: An immune-suppressing pill used after transplants and, off label, in membranous nephropathy. It was the comparison treatment in the MAJESTY trial of Gazyva.
tafamidis: A once-daily transthyretin stabilizer, sold in the US as Vyndamax, that holds the TTR protein together so it cannot misfold and form amyloid deposits in the heart.
Takhzyro: Brand name for lanadelumab, a preventive injection given every two weeks to reduce hereditary angioedema attacks.
taldefgrobep alfa: A myostatin inhibitor developed by Biohaven. Its Phase 3 trial in spinal muscular atrophy did not meet its primary endpoint. Biohaven is exploring the drug in other indications including obesity.
targeted therapy: Treatment that targets specific molecules involved in disease progression.
tau protein: A protein that accumulates abnormally in several neurodegenerative diseases including Alzheimer disease.
tebapivat: A second pyruvate kinase activator from Agios, dropped from sickle cell development in July 2026.
Tegsedi: Brand name of inotersen, a weekly antisense TTR silencer approved in 2018 with a boxed warning for platelet drops and kidney inflammation. The FDA lists it as discontinued.
telangiectasia: Small, dilated blood vessels visible near the surface of the skin or mucous membranes, appearing as fine red or purple lines.
telitacicept: A drug that blocks BAFF and APRIL, 2 signals that keep B cells and antibody-making plasma cells alive. It is approved in China and in Phase 3 testing for generalized myasthenia gravis in the United States.
Tepezza: The brand name for teprotumumab, the first and only FDA-approved medication for thyroid eye disease. Given as 8 infusions over about 5 months, it reduces eye bulging rather than only calming inflammation.
teprotumumab: An antibody (brand Tepezza) that blocks IGF-1R, the first drug approved for thyroid eye disease.
teratogenic: Capable of causing birth defects if taken during pregnancy. Teratogenic drugs require strict pregnancy prevention measures.
tetramer: A unit built from 4 copies of a protein. Transthyretin travels in the blood as a tetramer, and in ATTR amyloidosis the tetramer comes apart and the loose pieces misfold.
tetrathiomolybdate: A copper-binding agent that selectively removes copper from albumin and tissues. The investigational Wilson disease drug ALXN1840 (bis-choline tetrathiomolybdate) belongs to this class.
TFPI: Tissue factor pathway inhibitor, a natural brake on the start of clotting. Blocking it is how Hympavzi and Alhemo prevent bleeds in hemophilia A and B.
thalassemia: An inherited blood disorder in which the body makes too little hemoglobin, causing anemia. Severe forms need lifelong transfusions or gene therapy.
therapeutic: Relating to the treatment or healing of a disease or condition.
thrombocythemia: Too many platelets in the blood, as in essential thrombocythemia, a rare bone marrow disorder that raises clotting risk.
thrombocytopenia: Abnormally low platelet count, which increases bleeding risk.
thrombocytosis: An abnormally high number of platelets in the blood, which can increase the risk of blood clots.
thrombophlebitis: Inflammation of a vein caused by a blood clot, typically in the legs, causing pain, swelling, and redness.
thrombopoietin: A hormone that stimulates the bone marrow to produce platelets (blood cells needed for clotting).
thrombosis: A blood clot forming inside a blood vessel, which can block blood flow to organs or limbs.
thrombotic microangiopathy: Damage to the smallest blood vessels from tiny clots, which can injure the kidneys and destroy red blood cells and platelets. It appeared in a few people who received high doses of aPCC while on Hemlibra.
thrombus: A blood clot that forms inside a blood vessel, which can block blood flow and cause serious problems.
thymectomy: Surgical removal of the thymus gland, sometimes performed to treat myasthenia gravis or thymic tumors.
thymoma: A tumor of the thymus gland, which is associated with myasthenia gravis in some cases.
thyroid eye disease: An autoimmune condition, usually linked to Graves disease, where tissue behind the eye swells and pushes the eyeball forward. It causes bulging, pressure, double vision and in severe cases threatens sight.
thyroidectomy: Surgical removal of all or part of the thyroid gland.
thyroiditis: Inflammation of the thyroid gland, which can cause it to produce too much or too little thyroid hormone.
thyrotoxicosis: The state of having too much thyroid hormone acting on the body: fast heart rate, sweating, weight loss, muscle wasting. In MCT8 deficiency it affects the body but not the brain, so doctors call it peripheral thyrotoxicosis.
tirabrutinib: A BTK-blocking pill approved in Japan for primary central nervous system lymphoma, under FDA review in the US.
tiratricol: A modified form of the thyroid hormone T3 that can enter cells without the MCT8 transporter. Sold as Emcitate for MCT8 deficiency, approved in the EU in February 2025 and by the FDA on September 28, 2026. Also called Triac.
tissue factor pathway inhibitor: TFPI, a natural protein that puts a brake on the start of blood clotting. Hympavzi and Alhemo block it so people with hemophilia A or B can form clots without the factor they are missing.
tissue-agnostic: An approach to cancer treatment based on the specific genetic mutation driving the tumor, regardless of where in the body the cancer originated.
titanium dioxide: A mineral pigment that makes pills white or opaque. Not animal-derived.
titration: Gradual, stepwise adjustment of a drug's dose, usually upward from a low start, to find the amount that works with side effects the patient can live with.
TKI: Tyrosine Kinase Inhibitor, a type of targeted cancer drug that blocks enzymes driving cell growth.
TMPRSS6: A gene that helps control hepcidin, the body's master iron-regulating hormone. Silencing TMPRSS6 is a strategy in blood disorders like polycythemia vera.
tocilizumab: A monoclonal antibody (brand name Actemra) that blocks the IL-6 receptor. Used in rheumatoid arthritis, giant cell arteritis, systemic JIA, and other inflammatory conditions.
tolebrutinib: A brain-penetrant BTK inhibitor for progressive multiple sclerosis, approved in the EU as Cenrifki but not FDA-approved after a 2025 rejection over liver risk.
tonsillectomy: Surgical removal of the tonsils, sometimes performed for recurrent infections or obstructive sleep problems.
total thyroidectomy: Complete surgical removal of the thyroid gland, the standard treatment for medullary thyroid cancer.
toxicit: Side effects or harm caused by a treatment.
toxicity: Harm a substance can cause to the body. In drug development, toxicity studies in animals and humans map what doses cause what harms, and to which organs.
TPP1 enzyme: Tripeptidyl Peptidase 1, the enzyme missing in CLN2 Batten disease. Brineura replaces this enzyme through infusions directly into the brain.
transaminase: Liver enzymes. Elevated levels can indicate liver damage or disease.
transaminitis: Elevated liver enzymes (transaminases) in the blood, indicating liver inflammation or damage.
transfusion: Receiving donated blood or blood products (red cells, platelets, plasma) through an IV line.
transgene: A gene that has been transferred from one organism to another, often used in gene therapy to deliver a working copy of a faulty gene.
transition fill: A one-time temporary supply, usually 30 days, that a Medicare drug plan must give in the first 90 days of a new plan for a drug you already take that the plan doesn't normally cover.
translocation: A rearrangement where part of one chromosome breaks off and attaches to a different chromosome, sometimes creating cancer-driving gene fusions.
transplantation: Surgical placement of a healthy organ or tissue from a donor into a patient.
transsphenoidal surgery: Surgery performed through the nose and sinuses to remove a pituitary tumor. The first-line treatment for acromegaly.
transthyretin amyloid cardiomyopathy: A progressive heart disease caused by misfolded transthyretin (TTR) protein depositing as amyloid in the heart muscle, weakening its ability to pump.
transverse myelitis: Inflammation across a section of the spinal cord that causes weakness, numbness and bladder or bowel problems below that level.
TRAP degrader: Targeted Removal of Aberrant Proteins. Biohaven's proprietary platform for creating bispecific molecules that selectively tag disease-causing proteins for removal through the liver's natural clearance receptors.
trastuzumab: An antibody (brand Herceptin) that blocks the HER2 growth receptor on cancer cells.
treatment response: How well a patient's disease reacts to a given treatment.
treatment-emergent adverse event: A side effect that appears or worsens after a patient starts receiving the study treatment.
Triac: The research name for tiratricol, short for triiodothyroacetic acid, a thyroid hormone analog that bypasses the MCT8 transporter.
triage: The process of sorting patients by the severity of their condition to determine who gets treated first in an emergency department.
Trikafta: Brand name for elexacaftor/tezacaftor/ivacaftor, a triple-combination CFTR modulator therapy that treats the underlying cause of cystic fibrosis in most patients.
trinucleotide repeat: A section of DNA where three nucleotides are repeated many times, which can expand and cause disease.
triple seronegative: Testing negative for all 3 known myasthenia gravis antibodies: AChR, MuSK and LRP4. About 10% of people with generalized myasthenia gravis are in this group.
trisomy: Having three copies of a chromosome instead of the normal two. Down syndrome (trisomy 21) is the most well-known example.
TRK fusion: A gene rearrangement producing an overactive TRK protein that drives cancer. Detected by specialized genetic testing.
TRK inhibitor: A targeted drug that blocks the abnormal TRK fusion protein, stopping cancer growth. Works across different cancer types with TRK fusions.
trofinetide: An oral drug (brand Daybue), the first FDA-approved treatment for Rett syndrome.
Trop2: Trophoblast cell surface antigen 2. A protein found on the surface of many solid tumor types. Used as a target for antibody-drug conjugates that deliver chemotherapy directly to cancer cells.
tropomyosin receptor kinase: The protein encoded by NTRK genes. When fused with another gene, it becomes permanently active and drives cancer growth.
troponin: A heart muscle protein that leaks into the blood when heart cells are damaged. Raised levels are one of the yearly checks for TTR carriers.
troriluzole: A glutamate modulator developed by Biohaven for spinocerebellar ataxia. The FDA issued a Complete Response Letter in 2025, declining approval. It works by reducing excess glutamate signaling that damages nerve cells.
trough level: The lowest concentration of a drug in the blood, measured just before the next dose. Used to ensure the drug stays at a therapeutic level.
Truseltiq: Brand name for infigratinib, an FGFR-blocking pill approved for bile duct cancer in 2021 and withdrawn from that use in 2024.
Trutakna: Brand name of atacicept from Vera Therapeutics, a once-weekly home injection that blocks BAFF and APRIL. It received FDA accelerated approval for IgA nephropathy on July 7, 2026.
tryptase: An enzyme made by mast cells. Serum tryptase level is a key blood test for diagnosing and monitoring mastocytosis.
tryptophan: An essential amino acid used by the body to produce serotonin (a mood regulator) and melatonin (a sleep regulator).
TSH: Thyroid-stimulating hormone — a blood test measuring thyroid function. Some acromegaly medications can affect TSH levels, requiring monitoring.
TTR silencer: An injected RNA drug that destroys the liver's instructions for making transthyretin, lowering blood TTR by about 80%. Patisiran, vutrisiran and eplontersen are the approved silencers.
TTR stabilizer: A pill that binds the transthyretin protein and holds its 4-part cluster together so it cannot fall apart and misfold into amyloid. Tafamidis and acoramidis are the approved stabilizers.
tuberculosis: A bacterial infection, usually of the lungs. Many trials of immune-suppressing drugs screen for dormant tuberculosis before enrollment because treatment can reactivate it.
tubular: Relating to the tiny tubes in the kidneys that process filtered fluid into urine, reabsorbing useful substances and removing waste.
tularemia: A rare bacterial infection spread by ticks, rabbits, and contaminated water, causing fever and swollen lymph nodes.
tumor burden: The total amount of cancer in the body, often estimated by tumor size, number of tumors, or blood marker levels.
tumor marker: Substances (often proteins) produced by cancer cells or the body in response to cancer, measured in blood to track disease.
tunnel vision: Loss of peripheral (side) vision while central vision remains, creating a narrowing field of view. Characteristic of advancing RP.
TYK2: Tyrosine Kinase 2. An enzyme in the JAK-STAT signaling pathway involved in immune and inflammatory responses. Drug targets blocking TYK2 are being developed for autoimmune and neuroinflammatory conditions.
tyrosine kinase inhibitor: A targeted drug that blocks tyrosine kinases, enzymes that tell cells to grow. Used in cancers driven by overactive kinase signaling.
tyrosinemia: A rare inherited disorder where the body cannot break down the amino acid tyrosine, damaging the liver and kidneys without treatment.

U

UAS7: Urticaria Activity Score over 7 days: a patient diary score of daily itch and hive counts, from 0 to 42, used to measure chronic hives in clinical trials.
UBE3A: The gene silenced or lost in Angelman syndrome. Everyone inherits two copies, but in brain cells only the mother's copy is active, so losing it leaves neurons with no working UBE3A at all.
ublituximab: A B cell-depleting antibody (brand BRIUMVI) that targets CD20, approved for relapsing multiple sclerosis and being tested in myasthenia gravis.
ULN: Upper Limit of Normal — the highest value considered normal for a lab test.
ultra-rare: Affecting an extremely small number of people, often fewer than 1 in 100,000 globally.
umbralisib: A lymphoma pill (former brand Ukoniq) that TG Therapeutics voluntarily withdrew from the market in 2022 after a trial showed a survival imbalance.
umbrella trial: A trial testing multiple drugs within a single disease, with patients selected by specific biomarkers.
unilateral: Affecting only one side of the body.
unmet medical need: A disease where current treatments are inadequate, ineffective, or don't exist.
unresectable: Cannot be safely removed with surgery.
UPCR: Urine Protein-to-Creatinine Ratio, a test that measures how much protein is leaking from your kidneys. Used to monitor kidney disease progression.
upper airway: The nose, throat, and windpipe. In some storage disorders, tissue buildup narrows these passages and causes breathing difficulty.
upper limit of normal: The top of the normal range for a lab test at a given lab. Trial rules for liver tests are written as multiples of it, such as 3 times the upper limit of normal.
upper motor neuron sign: Clinical findings indicating damage to nerve fibers in the brain or spinal cord.
uremia: A dangerous buildup of waste products in the blood when the kidneys can no longer filter them, requiring dialysis.
urinary GAG: A urine test measuring glycosaminoglycan levels. Elevated urinary GAGs help diagnose MPS diseases and monitor treatment response.
urticaria pigmentosa: Brown or reddish-brown skin spots that itch or swell when rubbed, the most common skin finding in mastocytosis.
uveitis: Inflammation of the middle layer of the eye (uvea), which can cause eye pain, redness, and vision problems.

V

vagus nerve stimulation: A treatment for hard-to-control seizures in which a small implanted device sends regular pulses to the vagus nerve in the neck.
validated surrogate endpoint: A stand-in measurement that regulators have formally agreed predicts real patient outcomes well enough to approve a drug on. Proteinuria became one for IgA nephropathy in 2019, which let companies run 9-month trials instead of 10-year ones.
valoctocogene roxaparvovec: A gene therapy (brand name Roctavian) for severe hemophilia A that delivers a working copy of the Factor VIII gene via an AAV5 vector. BioMarin withdrew it from the market in February 2026, so it is no longer available to new patients.
Valsalva: A breath-holding, bearing-down maneuver done during an echo. It briefly reduces blood filling the heart, which unmasks or worsens outflow obstruction, so it is used to measure the provoked gradient.
valve disease: Damage to the heart valves that affects blood flow. In storage disorders, material buildup can thicken and stiffen heart valves.
vandetanib: A multi-kinase inhibitor (brand name Caprelsa) approved for advanced medullary thyroid cancer.
variant of uncertain significance: A genetic change whose effect on health is not yet known. More research is needed to determine if it causes disease.
vascular attenuation: Narrowing of the blood vessels in the retina, a common finding in retinitis pigmentosa that indicates reduced blood flow to the retina.
vasculature: The network of blood vessels (arteries, veins, capillaries) in a particular organ or throughout the body.
vasculitis: Inflammation of blood vessels that can restrict blood flow and damage organs.
vaso-occlusion: Blockage of blood vessels by sickled red blood cells, causing pain and tissue damage in sickle cell disease.
vaso-occlusive: Relating to the blockage of blood vessels, particularly by sickled red blood cells in sickle cell disease.
vaso-occlusive crisis: A painful episode in sickle cell disease where sickled red blood cells block small blood vessels, cutting off blood flow and causing severe pain.
vasopressin: A hormone (also called antidiuretic hormone/ADH) that helps the kidneys retain water and regulates blood pressure.
vector: This word carries 2 different meanings in medicine. In infectious disease, a vector is an organism such as a tick or mosquito that carries a pathogen from one host to another without being made ill by it. In gene therapy, a vector is the delivery vehicle, usually a modified virus, that carries a working gene into a patient's cells.
VEGFR-2: A receptor on blood vessel cells that tumors use to grow the new blood supply they need. Drugs that block VEGFR-2 aim to starve tumors of blood flow.
velaglucerase alfa: An enzyme replacement therapy (brand name VPRIV) for type 1 Gaucher disease that supplies the missing glucocerebrosidase enzyme.
venglustat: Sanofi's once-daily pill designed to reach the brain and slow production of the fatty substance that builds up in Gaucher disease. Under FDA priority review for Gaucher disease type 3, with a decision due November 25, 2026.
ventricle: The lower chambers of the heart that pump blood out to the body (left) and lungs (right). Also refers to fluid-filled spaces in the brain.
ventricular dysfunction: When the heart's lower chambers do not pump or fill properly, reducing the heart's ability to circulate blood.
Vimizim: Brand name for elosulfase alfa, an enzyme replacement therapy approved for mucopolysaccharidosis type IVA (MPS IVA, Morquio A syndrome). Made by BioMarin.
vincristine: A chemotherapy drug commonly used in cancer treatment. Vincristine is highly neurotoxic and can rapidly worsen CMT, so it should be avoided in CMT patients whenever possible. Patients should flag CMT on every medical record.
vitiligo: A condition where the skin loses its pigment cells, causing smooth white patches on various parts of the body.
Vitrakvi: Brand name for larotrectinib, an FDA-approved targeted therapy for cancers driven by NTRK gene fusions.
vitreous: The clear, gel-like substance filling the inside of the eye between the lens and the retina.
VlsE: Variable major protein-like sequence, expressed; a Borrelia protein used in modern immunoassays to detect Lyme disease antibodies.
Voice of the Patient: A report patient groups send the FDA after a patient-focused drug development meeting, summarizing what patients said about living with a disease and what they want from treatment.
voretigene neparvovec: A gene therapy (brand name Luxturna) that delivers a working copy of the RPE65 gene to retinal cells to treat inherited vision loss.
vosoritide: A C-type natriuretic peptide analog (brand name Voxzogo) that counteracts overactive FGFR3 signaling to restore bone growth in children with achondroplasia.
Voxzogo: Brand name for vosoritide, a C-type natriuretic peptide analog approved by BioMarin for achondroplasia (a form of dwarfism) in children with open growth plates.
Voyxact: Brand name of sibeprenlimab from Otsuka, an injection that blocks APRIL alone. It received FDA accelerated approval for IgA nephropathy in November 2025.
VPRIV: Brand name for velaglucerase alfa, an IV infusion enzyme replacement therapy for type 1 Gaucher disease.
VUS: Variant of Uncertain Significance, a genetic change that has not yet been classified as harmful or harmless.
vutrisiran: An RNA interference drug (brand name Amvuttra) that stops the liver from making transthyretin. A clinic injection every 3 months, approved for hereditary ATTR polyneuropathy (2022) and ATTR cardiomyopathy (2025).
Vyndamax: The 61 mg once-daily capsule form of tafamidis, Pfizer's TTR stabilizer for ATTR cardiomyopathy. One Vyndamax capsule equals four 20 mg Vyndaqel capsules.
Vyndaqel: The original brand of tafamidis, taken as four 20 mg capsules a day for ATTR cardiomyopathy. Drugs@FDA now lists it as discontinued in the US; Vyndamax, one 61 mg capsule a day, is the form sold.

W

Wainua: Brand name of eplontersen, an antisense TTR silencer that patients inject themselves once a month. Approved in December 2023 for hereditary ATTR polyneuropathy; not approved for the heart form.
warfarin: A commonly prescribed blood-thinning medication that prevents blood clots by interfering with vitamin K-dependent clotting factors.
washout: A planned period off a previous medication so it clears the body before starting a trial or a new treatment.
washout period: A waiting period to let a previous treatment leave your body before starting a new one.
Western blot: A lab test that separates proteins into bands on a strip to identify specific antibodies. Previously the second step in Lyme disease diagnosis.
wheezing: A whistling sound when breathing, caused by narrowed airways. Common in asthma and other lung conditions.
whole lung lavage: The standard treatment for pulmonary alveolar proteinosis: under general anesthesia, one lung is repeatedly filled with salt water and drained to wash out built-up surfactant while the other lung breathes.
wholesale acquisition cost: A drug's US list price, set by the manufacturer, before the confidential discounts and rebates that insurers negotiate. Often shortened to WAC.
wholesale list price: The manufacturer's published price to wholesalers before any insurance negotiation, rebates, or assistance programs. Almost nobody pays this number directly, but it anchors coverage decisions.
wild-type ATTR: ATTR amyloidosis in a person with a normal TTR gene, where the protein becomes unstable with age. It mostly affects the heart, in men over 70.
Winrevair: Brand name for sotatercept, the first activin signaling inhibitor approved for pulmonary arterial hypertension.
WOCBP: Women Of Child-Bearing Potential — women who could become pregnant.
women of child-bearing potential: Women who are physically able to become pregnant.

X

X-linked: A pattern of genetic inheritance where the mutated gene sits on the X chromosome. Males have only one X, so they are typically affected more severely than females. Fabry disease and Duchenne muscular dystrophy both follow this pattern.
X-linked dominant: An inheritance pattern where one copy of a disease gene on the X chromosome causes disease.
X-linked recessive: An inheritance pattern where males are typically affected and females are carriers.
xanthomatosis: Fatty deposits building up in tendons, brain, and other tissues, as in cerebrotendinous xanthomatosis, a treatable genetic disorder.
xerophthalmia: Chronic dry eyes caused by reduced tear production. A primary symptom of Sjögren's syndrome.
xerostomia: Chronic dry mouth caused by reduced saliva production. A primary symptom of Sjögren's syndrome that increases risk of dental cavities and oral infections.

Y

Yorvipath: Brand name for palopegteriparatide, an Ascendis Pharma therapy for hypoparathyroidism that provides sustained parathyroid hormone replacement.
Yuviwel: Brand name for navepegritide, an Ascendis Pharma therapy for achondroplasia that works by modulating C-type natriuretic peptide signaling to promote bone growth.

Z

Z-AAT: The misfolded form of alpha-1 antitrypsin made from the Z variant. It gets stuck inside liver cells, which can damage the liver, instead of reaching the lungs.
zanidatamab: A bispecific antibody (brand name Ziihera) that grabs the HER2 growth receptor at 2 sites. FDA-approved in November 2024 under accelerated approval for HER2-positive biliary tract cancer that has already been treated.
Zemaira: CSL Behring's brand of alpha-1 proteinase inhibitor for AATD augmentation, approved in 2003 and the product tested in the RAPID trial.
zilucoplan: A once-daily self-injected complement C5 blocker (brand Zilbrysq) approved for adults with anti-AChR antibody-positive generalized myasthenia gravis.
zilurgisertib: A once-daily oral ALK2 inhibitor (brand name Atebrioz) approved by the FDA on September 25, 2026 for people 12 and older with FOP, to reduce the abnormal bone formation of the disease.
Zolgensma: Brand name for onasemnogene abeparvovec, a one-time gene therapy for spinal muscular atrophy (SMA) in children under two years old.
zurletrectinib: A next-generation TRK inhibitor for NTRK fusion cancers, approved in China in December 2025. It has not been filed for approval in the US.

Missing a term you ran into? Email jason@trialfriend.com and it will be added with a plain-language definition. For deeper explanations of how trials work, start with the Learn guides.