Whereas normally tau is in equilibrium between the free and microtubule-associated forms, under some conditions it aggregates into neurofibrillary tangles and other types of intracellular inclusions, and these aggregates are believed to play a key role in the pathogenesis of Alzheimer's disease and several other neurodegenerative disorders (2, –, 4). The gene discussed is MAPT; the disease is early-onset autosomal dominant Alzheimer disease.