The 3 isoforms of ApoE—ApoE2, ApoE3, and ApoE4—differ by 1 or 2 amino acid residues but have markedly different effects on AD risk: ApoE2, which has an allele frequency of ~ 8%, is known to be neuroprotective, whereas ApoE4 has a prevalence of approximately 14% and is one of the strongest risk factors for AD [5–7]. Here, APOE is linked to Alzheimer disease.