APOE and post-traumatic stress disorder: APOEε2, the least common of the 3 APOE alleles, has a protective effect for AD and risk and protective effects for several other neurological diseases including stroke, cerebral amyloid angiopathy, posttraumatic stress disorder, age-related macular degeneration, progressive supranuclear palsy, and argyrophilic grain disease.8 Because APOEε2 is less common, we had less power to detect an association.