Although AD patients who are APOE ε4 carriers have greater tau pathology compared to noncarriers5, the APOE ε4 genotype is not associated with primary age-related tauopathy (PART), which has medial temporal neurofibrillary pathology but no (“definite PART”) or only minimal (“possible” PART) Aβ deposition38, implying that apoE4 might only impact tau pathology in the presence of amyloid. This evidence concerns the gene MAPT and Alzheimer disease.