49ers' Chris Borland retiring at 24, wary of head trauma
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San Francisco's Borland joins recent list of early retirees in NFL
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NFL rookie Chris Borland retires over head trauma concerns
San Francisco 49ers rookie LB Chris Borland retires at age 24 over
NFL Player Chris Borland Retires Due to Concussion Risk
49ers Wunderkind Retires After 1 Year, Brings Concussion Issue