For the first time, scientists have seen the ravages of repeated concussions in the brains of living people. Brain scans of five retired National Football League players reveal unusual deposits of an ominous protein called tau that used to be identifiable only through autopsy. Tau is a hallmark of chronic traumatic encephalopathy, a disease that causes the gradual appearance of mental and emotional problems in people who have experienced repeated brain trauma.The new method of looking for tau relies on a chemical that affixes to the protein and serves as a beacon, lighting up in PET scans of the brain. More tau turned up in the brains of former NFL players than in other men, particularly in deep brain areas called the thalamus and the amygdala. The more concussions a player sustained, the more tau scientists found, Gary Small of UCLA and colleagues report online January 22 in the American Journal of Geriatric Psychiatry . This ability to track CTE in players may help researchers understand what happens in brains that get repeatedly hit over the course of a professional football career.