The mean age of clinical onset is 68 years in ε4 homozygotes, 76 years in ε4 heterozygotes, and 84 years in ε4 non-carriers, indicating that APOE ε4 dramatically increases the risk of AD development with an earlier age of onset in a gene dose-dependent manner (Corder et al., 1993; Rebeck et al., 1993). This evidence concerns the gene APOE and Alzheimer disease.