MAPT and age: Two main hypotheses currently exist for integrating such cases nosologically: One favors unity, i.e., a continuum from tau+/Aβ− to tau+/Aβ+; the PART concept posits a duality of processes, AD vs ‘aging’, with age-associated tauopathy (tau+/Aβ-) defining PART and considered to be a process different from AD-associated tau+/Aβ+.