APOE and Alzheimer disease: In Alzheimer’s Disease Neuroimaging Initiative (ADNI), plasma Aβ fails to differentiate AD patients from control individuals and amyloid-positive from amyloid-negative individuals, although a weak positive relationship between plasma Aβ40/Aβ42 ratio and Aβ ligand retention on PET was observed in APOE ε4-negative subjects only14.