Teenage Derby County defender Mark O'Brien has signed a four-year deal.
The 18-year-old Republic of Ireland Under-19 international has seized his chance in the first team this season, featuring in all seven Rams games.
O'Brien has started six matches at centre-half, starting in place of the injured Shaun Barker in five of the club's six Championship matches.
Manager Nigel Clough told the club website: "He has been playing well and deserves his contract."
O'Brien, who will now stay at Pride Park until the summer of 2015, had to undergo heart surgery in Novemer 2009.
And Clough has been impressed with how he has come back: "He has had to overcome some challenging hurdles but his attitude has been exemplary.
"He has taken his chance because of the injuries.
"He has shown to everybody he is an honest, brave and committed player who will put his head where some players would not even put their feet."