Strong evidence from clinical and basic research indicates that the APOE ε4 allele is associated with an increased risk of AD (Corder et al., 1993), while the APOE ε2 allele is linked to a decreased risk compared to the more common APOE ε3 allele (Bu, 2009; Corder et al., 1994; Farrer et al., 1997). This evidence concerns the gene APOE and Alzheimer disease.