The apolipoprotein E (APOE) gene is the strongest genetic risk factor for sporadic AD3; carrying 1 APOE-ε4 (APOE4) allele increases risk 3–4-fold and 2 alleles increase risk 8–12-fold compared to the common APOE-ε3 allele4. This evidence concerns the gene APOE and Alzheimer disease 3.