The best established genetic risk factor for sporadic AD is the ε4 allele of the APOE (apolipoprotein E) gene – compared to individuals with the most common genotype (ε3/ε3), individuals with one copy of the ε4 allele are three times more likely to develop AD, while those with two copies of the ε4 allele are 10–15 times more likely to develop AD (Corder et al., 1993). This evidence concerns the gene APOE and Alzheimer disease.