These findings indicate that plasma Aβ42/Aβ40 may be useful for screening APOE-ε4 carriers (such as Alzheimer’s Prevention Initiative (API) Generation Study) as the potential participants for anti-AD clinical trials with neurodegeneration or cognitive decline as the ending points, whereas its application may be limited in non-demented elderly adults without APOE-ε4 allele. The gene discussed is APOE; the disease is Alzheimer disease.