For neocortical tau SUVR and PACC change, EXT was the better predictor in earlier Aβ stages (while Aβ spread) while SUVR was superior later (after Aβ was widespread).<h4>Conclusions</h4>EXT is a robust, generalizable PET metric that detects Aβ before global positivity and early Aβ-related changes in tau and cognition, supporting its relevance for trial enrichment and early therapeutic monitoring in AD prevention trials. The gene discussed is MAPT; the disease is Alzheimer disease.