In fact, people with one ε4 allele have about a three-fold increased risk of Alzheimer’s disease, and those with two ε4 alleles have about a 15-fold increased risk, compared with those with the most common genotype, APOE ε3ε3 (Bettens et al., 2013; Sims et al., 2020). Here, APOE is linked to early-onset autosomal dominant Alzheimer disease.