APOE confers susceptibility in a dose-dependent manner: relative to an individual with the most common APOE ε3/ε3 background, individuals heterozygous for APOE ε4 (ε4/ε3) are subjected to a 2–4 times greater risk of developing AD and individuals homozygous for APOE ε4 (ε4/ε4) rise to an 8–15 times greater chance of developing AD (Corder et al, 1993; Farrer et al, 1997; Genin et al, 2011; Holtzman et al, 2012). The gene discussed is APOE; the disease is Alzheimer disease.