APOE and Alzheimer disease: APOE is one of a cluster of genes in the Chr 19q13.32 genomic region that affect AD risk, that also includes PVRL2, TOMM40, and APOC1. There are three common forms of APOE, distinguishable by the identity of amino acids at positions 112 and 158 that are determined by two closely linked SNPs in the APOE gene: rs429358 and rs7412, that result in the expression of three alternative protein isoforms: APOE ε2, which possesses cysteine residues at both positions, APOE ε3, which possess cysteine-112 and arginine-158, and APOE ε4, which possesses arginine residues at both positions.