As Alzheimer disease (AD) research moves to intervene in presymptomatic phases of the disease, we must develop outcome measures sensitive to the earliest disease-related changes.A preclinical stage of AD (PCAD) has been described in which subjects show no overt clinical manifestations of AD but demonstrate significant AD pathology at autopsy.The genetic biomarker, delineating the extent and distribution of amyloid-beta (Aβ) deposits in the brain, that can be useful in identifying cognitively normal people who are at enhanced risk of developing PCAD.