APOE ε4 carriers show elevated risk of MCI and Alzheimer’s disease, with one allele increasing risk two- to three-fold and homozygosity conferring up to a ~15-fold increase, as well as earlier age at onset; APOE ε4 is also associated with greater hippocampal atrophy in individuals with MCI (Hu et al., 2017; Jefferson et al., 2015; C.-C. Here, APOE is linked to early-onset autosomal dominant Alzheimer disease.