NFL star Chris Borland quits over head injury concerns - CBS News
49ers Chris Borland retires amid head trauma worries
NFL standout quits, citing brain damage risk - WHYY
49ers star Borland, 24, retires over health concerns
Twenty-Four-Year-Old 49ers Linebacker Retires, Citing Fears of Brain Injury, East Bay Express
How many millions did 49ers' Chris Borland give up by retiring at 24?
San Francisco 49ers linebacker Chris Borland retires due to health fears, NFL News
Chris Borland retires: NFL must improve player safety - Sports Illustrated
49ers Wunderkind Retires After 1 Year, Brings Concussion Issue Again to Fore
24-year-old NFL player quits over concussion worries