MAPT and Alzheimer disease: Although this remains to be clarified in the case of prion protein, it is strongly suggested that the dimer may be the minimal unit in tauopathies, because an equimolar ratio of 3R and 4R tau isoforms is integrated into the unique filamentous structure (PHF) in AD, whereas fibrils composed of only 3R tau or only 4R tau are deposited in other tauopathies.