A rising star of American football has retired after just one season because he fears his health could suffer from the long-term effects of concussion.
Chris Borland, a linebacker for the San Francisco 49ers, quit after talking to concussion researchers and his family.
His four-year deal with the 49ers was reportedly worth around $3m (£2.03m).
The 24-year-old Borland, who suffered two prior concussions before joining the 49ers, was among the top rookies in the NFL last season.
He finished with 107 tackles and two interceptions.
The issue of brain injuries in American football is a hot topic.
A class-action lawsuit involving thousands of former players, filed in 2012, contended that the NFL hid the dangers of brain injury among players while profiting from the sport's violent physical contact.
In February, a US judge refused to accept a proposed settlement between the NFL and the players, saying payment should be expanded for some players and families among other concerns.
Borland becomes one of the most prominent NFL players to quit because of concerns about brain injuries.
Last year Seattle Seahawks receiver Sidney Rice retired at the age of 27 over fears for his long-term health after absorbing so many blows to the head.
Speaking to ESPN's Outside the Lines, Borland said: "I just want to do what's best for my health. From what I've researched and what I've experienced, I don't think it's worth the risk."
San Francisco general manager Trent Baalke said the news was "unexpected", while the NFL insisted "football has never been safer".
Jeff Miller, the NFL's senior vice president of health and safety policy, added he respected Borland's decision and accepted there was still work to do. But he added: "We continue to make progress with rule changes, safer tackling techniques at all levels of football, and better equipment, protocols and medical care for players."
Borland, a history graduate at the University of Wisconsin, plans to further his education and pursue a career in sports management.