APOE and Alzheimer disease: Patients with an atypical Alzheimer’s disease variant, such as early-onset Alzheimer’s disease or a missing APOE E4 genotype show only mild or no hippocampal atrophy, but, instead, present with considerable posterior cortical atrophy, e.g., the precuneus region in early-onset Alzheimer’s disease (illustrated in Fig. 9) [6].