One gene called APOE comes in several different alleles, including APOE2, APOE3, and APOE4. APOE3 is neutral and is the most common allele (observed in 75% of the population); APOE2 reduces the risk of AD, but APOE4 is associated with an increased risk of AD119 and late onset of the disease16,120. Here, APOE is linked to Alzheimer disease.