APOE and early-onset autosomal dominant Alzheimer disease: Background: The current pilot study was designed to examine the association between hippocampal γ-aminobutyric acid (GABA) concentration and episodic memory in older individuals, as well as the impact of two major risk factors for Alzheimer’s disease (AD)—female sex and Apolipoprotein ε4 (ApoE ε4) genotype—on this relationship.