Tau protein tangles (or neurofibrillary tangles) are a hallmark of Alzheimer’s disease, but what role do they play? First, you need to understand how a disease like Alzheimer’s spread across the brain. Degenerative diseases including Alzheimer’s, Parkinson’s and other dementias are “prion-like,” meaning they happen because proteins in the brain misfold and change their structure. Once one protein misfolds, the same process can spread to other parts of the brain, causing many cells to misfold. That results in what we call tau tangles.