The substantial heterogeneity of AD from a genetic point of view emerged, for example, in recent genome-wide association studies revealing how the ∊4 allele of the apolipoprotein E (APOE) gene shows a dose-dependent relationship with increased risk of late-onset and sporadic cases of AD, while the inheritance of the ∊2 allele is protective, highlighting the possibility of stratifying AD patients based on their APOE genotype [32,50,51,52] (Table 1). The gene discussed is APOE; the disease is Alzheimer disease.