Researchers have long made the link between playing contact sports like football and developing the degenerative brain disease Chronic Traumatic Encephalopathy or CTE. But they’ve struggled to answer a nagging question: why doesn’t it impact all players?

“We certainly see that two individuals who have certain amounts of, say football play — one of them may go on to develop severe CTE pathology and another may have only mild disease,” says Dr. Jesse Mez, an assistant professor of neurology at Boston University School of Medicine. “There has to be another explanation above and beyond their contact sport exposure that explains this difference.”

Mez and his colleagues examined the brains of 86 former football players. All had evidence of CTE, but some had far more advanced cases. It was in the brains of the more severely impacted players that researchers discovered a variation of a gene linked to brain inflammation called TMEM106B.

Mez says it’s one of potentially hundreds of genetic variants that likely make some people more susceptible to CTE than others.

“Down the road we’ll be able to take this information, as well as future information that we learn, and combine it with other environmental information,” says Mez, “and based on that information together help people determine their risk.”

He hopes the findings may also lead to treatment that could slow down, or one day stop, the progression of the disease.