A stepwise decrease in plasma levels of apoE is observed from ε2 to ε3 to ε4, and recently three prospective studies reported that low plasma apoE levels were associated with high AD risk, independent of the ε2/ε3/ε4 polymorphism (Rasmussen et al., 2015; Wolters et al., 2016). Here, APOE is linked to Alzheimer disease.