MAPT and Alzheimer disease: Previous longitudinal PiB data estimated that the threshold for Aβ positivity was reached many years before dementia onset.7 We observed that tau changed shortly after Aβ positivity; however, we also observed high variability in tau change among individuals with high Aβ; those with rates of tau change similar to the low-Aβ group had stable cognition, highlighting that the Aβ-cognition delay may be variable and emphasizing the value of measuring tau to track disease progression in preclinical AD.