Adult human nerve cells contain tau protein, a phosphorylated microtubule-associated protein, that is hyperphosphorylated in the fetus and in patients with Alzheimer's disease. Hyperphosphorylation, which diminishes the microtubule-binding capacity of tau, destabilizes microtubules and may enhance the formation of paired helical filaments that constitute neurofibrillary tangles in Alzheimer's disease.