A potential explanation for the performance of tau PET to predict conversion to all-cause dementia may be 3-fold: (1) a significant proportion of individuals with AD dementia (ie, 3R/4R tauopathy) may have been part of this group, and tau aggregation is closely related to neuronal dysfunction and cognitive decline; (2) some individuals with primary 3R and 4R tauopathies may have been detected by tau PET; and (3) possible comorbid tauopathy may have had a synergistic impact on cognitive decline. This evidence concerns the gene MAPT and tauopathy.