The ε4 allele of the apolipoprotein E (APOE ε4) gene is the strongest known genetic risk factor for dementia and cognitive impairment, and it may represent roughly half of the genetic risk for neuropathologically based cognitive diseases (Raber et al., 2004; Caselli et al., 2010; Lim et al., 2018). The gene discussed is APOE; the disease is cognitive disorder.