APOE and Alzheimer disease: Of the 3 APOE isoforms, APOE ε4 is the strongest and most common genetic risk factor for late-onset AD as compared to the most common APOE isoform, APOE ε3, while the third isoform, APOE ε2, may increase risk for primary tauopathies (Corder et al., 1993; Jones and Rebeck, 2018; Zhao et al., 2018).