49ers' Chris Borland, 24, retires because of brain injury concerns
49ers Wunderkind Retires After 1 Year, Brings Concussion Issue
Borland retires at 24, citing health concerns
San Francisco 49ers rookie LB Chris Borland retires at age 24 over
San Francisco 49ers Chris Borland Retiring at 24, News
Chris Borland retires from 49ers after one year due to fears over
NFL: Chris Borland retiring at 24, wary of head trauma
Former 49ers LB Chris Borland says health more important than
Chris Borland: NFL player quits over concussion fears - BBC Sport