A protein that accumulates in the brain neurons of people with Alzheimer’s disease, creating entanglements and disrupting neuron activity. Tau is important to nerve cell functions and to maintenance the neuron’s cellular structure and integrity. The tau gene controls tau metabolism; it appears that a defect in this gene allows either excessive tau production or incomplete tau metabolism, leading to accumulations of tau deposits within neurons. Doctors can detect deposits only at autopsy after death. Some people with Parkinson’s disease also have tau deposits. Researchers are not certain whether the deposits indicates that such people also had the beginnings of, or undetected, Alzheimer’s disease at the time of death or that there are connections between dysfunctions with tau metabolism and that of the synuclein (a different brain protein) that composes the lewy body deposits in brain neurons that are characteristic of Parkinson’s disease. Scientists do suspect a connection as there appear to be overlaps in the disease processes of Alzheimer’s and Parkinson’s.