APOE and Alzheimer disease: Evidence suggests that heterozygous carriers of an ε4 allele are 3–4 times more likely to develop the AD than noncarriers (Colovati et al., 2020), so GWAS for the AD typically include APOE4 allelic dosage (i.e., the number of ε4 alleles in a subject’s APOE genotype) as a covariate (Mormino et al., 2016; Wang et al., 2021; Schneider et al., 2022).