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Apolipoprotein E (ApoE) is a well‑defined genetic risk factor for late‑onset Alzheimer’s disease (AD). The human APOE gene has three polymorphic alleles, E2, E3 and E4 that result in six different genotypes. Approximately half of AD patients carry the E4 allele (vs 14 % in the general population), with E4 dosage correlated with higher disease risk and lower age of onset. Differences between isoforms arise from two amino acids altering protein structure and lipid‑amyloid‑β interactions; circulating plasma and CSF ApoE levels are emerging as AD biomarkers relevant to brain amyloid pathology, complementing beta‑amyloid testing.