Alzheimer disease (AD) is a quintessential disease of aging. Onset before age 60 years is rare and is often attributable to one or more disease-causing genetic mutations. After age 60, the incidence of the disease doubles approximately every 5 years, at least until age 90.1 If one survives to age 100, the cumulative incidence of AD probably exceeds 75%.2,3 Yet AD is clearly heritable, with twin studies suggesting that perhaps 70% of variability in its predisposition results from genes.4 APOE typically accounts for about half of the total genetic influence.5 APOE , and probably other genes, alters susceptibility for late-onset AD by modifying age at onset.2