APOE and posterior cortical atrophy: Genes also influence AD: the apolipoprotein E (APOE) e4 allele is another important risk factor for atrophy patterns and age at onset; it is implicated in an earlier age at onset (Breitner et al., 1998, Corder et al., 1993), targeted hippocampal atrophy (Manning et al., 2014), and although it is a risk factor for 1 presentation of atypical early-onset AD: posterior cortical atrophy (PCA) (Schott et al., 2015), its effects are weaker than for typical AD.