A major hypothesis for how apoE4 affects the onset of AD contends that apoE promotes the aggregation of Aβ into amyloid plaques in the brain, either through impairing Aβ clearance [21,22], directly regulating the propensity of Aβ to form amyloid fibrils [23,24], or both mechanisms. The gene discussed is APOE; the disease is Alzheimer disease.