Patients carrying the APOE4 risk allele and having a first-degree family history of Alzheimer’s disease as well as homozygous APOE4 allele carriers showed a thinner cortex in six of the seven medial temporal subregions, including hippocampal cornu ammonis fields 2, 3, and the dentate gyrus. The gene discussed is APOE; the disease is early-onset autosomal dominant Alzheimer disease.