MAPT and Alzheimer disease: We tested this hypothesis in five cases of high ADNC with incidental PSP (“ADNC + PSPi”) pathology and found, first, that neuroanatomic distributions of 4R PSP and 3R/4R AD pathology are conserved in ADNC + PSPi and, second, that RT-QuIC can robustly distinguish the neuroanatomic distribution of 3R/4R versus 4R pathology in cases with mixed tau pathology, suggesting its utility as a tool to interrogate pathophysiology.