In humans, the gene exists in three allele variants called ε2, ε3, and ε4. In comparison to the APOE ε3/ε3, a single copy of the APOE ε4 allele results in a 3- to 4-fold increase in the risk for AD, and APOE ε4/ε4 results in a 9- to 15-fold increase (Liu et al., 2013; Yamazaki et al., 2019). The gene discussed is APOE; the disease is Alzheimer disease.