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). This evidence concerns the gene CCR5 and Alzheimer disease.