Glossary
Terms used on this site
Short definitions of the genetics, biomarker and trial terms that come up in APOE4 research.
- Allele
- One version of a gene. APOE has three common alleles: ε2, ε3 and ε4.
- Homozygote / heterozygote
- A homozygote carries two copies of the same allele (ε4/ε4). A heterozygote carries two different alleles (for example ε3/ε4).
- Penetrance
- The share of people with a genotype who develop the disease. ε4/ε4 shows high but incomplete penetrance.
- Amyloid-β (Aβ)
- A protein fragment that clumps into plaques between brain cells; the earliest known change in Alzheimer’s.
- Tau
- A protein that forms tangles inside neurons. Tau spread tracks closely with symptoms.
- Cerebral amyloid angiopathy (CAA)
- Amyloid deposited in the walls of brain blood vessels, which weakens them and raises the risk of bleeds. More common with ε4.
- Microbleed
- A tiny, usually silent bleed in the brain, seen on certain MRI scans.
- ARIA
- Amyloid-related imaging abnormalities: brain swelling (ARIA-E) or small bleeds and iron deposits (ARIA-H) seen on MRI, mainly with anti-amyloid antibodies. Risk is highest in ε4/ε4.
- Amyloid PET / tau PET
- Brain scans that measure amyloid plaques or tau tangles in living people.
- Plasma biomarkers
- Blood tests for Alzheimer’s-related proteins, such as p-tau217, Aβ42/40, NfL and GFAP.
- MCI
- Mild cognitive impairment: measurable decline in memory or thinking that does not yet disrupt daily life.
- CDR-SB, ADAS-Cog, iADRS
- Rating scales used as outcomes in Alzheimer’s trials to measure cognition and daily function.
- Preclinical Alzheimer’s
- Alzheimer’s brain changes, such as amyloid, without symptoms. Prevention trials enroll at this stage.
- Natural history study
- An observational study that follows people over time without an experimental treatment, to learn how a disease unfolds.
- Gene therapy
- Delivering genetic material, often with a harmless virus (AAV), to change what cells produce, for example adding APOE2.
- APOE3 Christchurch
- A rare APOE variant that appeared to protect a carrier of an early-onset Alzheimer’s mutation, pointing to APOE as a drug target.