APOE and post-traumatic stress disorder: The latter, at elevated levels, mimics the effects seen in post-traumatic stress disorder in the hippocampus, and negatively affects memory.60, 61 ApoE is encoded by the APOE gene, located on chromosome 19, which has three major alleles: ɛ2, ɛ3 and ɛ4, related to amino-acid substitutions (Arg and Cys) at positions 112 and 158 of the protein.