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A new Boston University study warns that amateur athletes in contact sports have a high risk for a rare brain disorder.

Chronic trauma encephalopathy, or CTE, is a little-understood brain disorder that affects thinking, memory, and behavior. It’s believed to be caused by repeated strikes against the head, and it’s only diagnosable through an autopsy.

The university did a study on more than 150 donated brains from athletes who died before the age of 30. They found that 40% of these athletes had the syndrome, whereas 1% of the general population has the disease.

The athletes primarily played football, soccer, and ice hockey. The youngest diagnosis came from a 17-year-old high school football player.

The syndrome is untreatable as of now, though experts are urging athletes to treat symptoms like suicidal thoughts or mood changes.