While human APOE4 heterozygotes exhibit an approximate 3-fold to 5-fold increase in risk of AD, APOE4 homozygotes have an even higher risk (Corder et al., 1993; Farrer et al., 1997; Mahley and Huang, 2012; Neu et al., 2017; Belloy et al., 2019). The gene discussed is APOE; the disease is Alzheimer disease.