ApoE is polymorphic with three major alleles, namely, APOE*ε2, APOE*ε3, and APOE*ε4. People with two APOE*ε4 alleles are at a 15 times higher risk of AD as compared to APOE*ε3 carriers, and APOE*ε2 allele carriers have protection against AD. Here, APOE is linked to Alzheimer disease.