Tau protein was a well-studied molecule before it was discovered in the Alzheimer neurofibrillary tangles. As a microtubule-associated protein (MAP), it continues to be of interest to microtubule biologists who have provided a rather rich knowledge about this protein. Recent work suggests that tau, a neuronal MAP, is capable of generating some features of an axonal shape and an axon-like organization of the cytoskeleton. The importance of tau in pathology stems from its relationship to Alzheimer paired helical filaments and dystrophic neurites. Tau was first believed to be a component of paired helical filaments based upon immunocytochemical grounds (1-6) and then conclusively demonstrated by protein chemical techniques (7-9). Most recently it was shown that bacterially expressed tau fragments from the microtubule-binding domain can self-assemble into paired helical filaments that resemble those from the Alzheimer brain (10).