APOE and Alzheimer disease: Overall, since low plasma apoE levels were associated with a higher risk of AD [12] and specifically the APOE ε4 genotype appears to contribute to this higher risk by its association with lower plasma apoE levels [10, 32], we hypothesized that the difference in AD risk between populations might be attributed to corresponding differences in their plasma apoE levels, hence individuals with higher AD risk exhibiting lower plasma apoE levels and vice versa.