APOE and Alzheimer disease: To test the hypothesis that the APOE-ε4 and APOJ-C risk alleles alter the structure of a brain region affected early in AD pathogenesis (Hyman et al., 1984; Gomez-Isla et al., 1996; Braak et al., 2011), we extracted gray matter volumetric measurements within the left and right entorhinal cortical regions of interest for each genotype group.