Alzheimer's disease (AD) is unique in medicine in that millions of people suffer from what appears to be the same form of disease, and unlike most other late-onset diseases, the genetic etiologies have been well identified. Three early onset forms of AD inherited as autosomal dominant traits account for less than 2% of prevalent AD. A major susceptibility locus, apolipoprotein E (APOE, gene; apoE, protein) is associated with risk and age of onset distributions for the common familial and sporadic late-onset AD. The identification of additional genetic susceptibility genes in the etiology of AD and the metabolic mechanisms leading to differences in age of onset and disease pathogenesis are active areas of current research.