APOE and Alzheimer disease: The Apolipoprotein E (APOEε4) allele appears to be a risk factor accounting for only 10%–25% of AD cases and not an invariant cause of AD, as more than half of AD cases do not have the high-risk E4 allele, indicating that other environmental or genetic factors may need to be concurrently acting with this allele to cause AD (Hyman et al., 1996; Lambert et al., 2013; Haines, 2018; Jiang et al., 2019).