For example, a genetic variant in the gene ABO causes the O blood type (Yamamoto et al., 1990), a variant in CCR5 renders cells resistant to HIV infection (Dean et al., 1996; Samson et al., 1996), and variants in the genes APOE (Corder et al., 1993) and TREM2 (Guerreiro et al., 2013; Jonsson et al., 2013) are among the strongest known genetic contributors to cardiovascular disease (APOE) and Alzheimer’s disease (AD, both APOE and TREM2). Here, APOE is linked to Alzheimer disease.