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Former NFL running back Ronnie Hillman died from a rare form of kidney cancer at age 31. His family confirmed his death in a statement on Thursday, Dec. 22.

“It is with heavy hearts that we announce the passing of our beloved son, brother & father, Ronnie K Hillman Jr,” the statement read. “Ronnie quietly and peacefully transitioned today in the company of his family and close friends. We as the family, appreciate the prayers and kind words that have already been expressed. We ask that you would give us time to process our feelings as we prepare to lay our precious RJ to rest.”

Hillman’s family announced he was diagnosed with renal medullary carcinoma in August, TMZ reported. Renal medullary carcinoma is a very rare, terminal, and aggressive form of kidney cancer. His health declined rapidly and he passed away in a hospice.

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Renal medullary carcinoma is difficult to treat. It spreads quickly, and is at end-stage by the time it is diagnosed.

Signs and symptoms of renal medullary carcinoma include blood in urine (hematuria), flank (back/side) pain, and unexplained weight loss.

The cancer is so rare that it mainly affects Black people with sickle cell trait or sickle cell disease. It isn’t clear if Hillman had either sickle cell trait or sickle cell disease.

Hillman was a standout running back at San Diego State, where he carried the ball 262 times for 1,532 yards and 17 touchdowns in 2010.

He played for the Broncos from 2012 through 2015, and helped the team win Super Bowl 50. He also played for the Minnesota Vikings, San Diego Chargers and Dallas Cowboys.