Chelsea have completed a £12m move for 19-year-old St Etienne centre-back Kurt Zouma.
The France Under-21 player has agreed a five-and-a-half-year deal with the Blues, but will stay at St Etienne on loan for the rest of the season.
Zouma has made 12 Ligue 1 appearances for the French side this season.
Born in Lyon, Zouma joined St Etienne as a 15-year-old in 2009 before turning professional two years later.
He has made 52 appearances for the club, scoring four goals.
St Etienne president Roland Romeyer had previously said the player would not move in the transfer window.
Meanwhile, Blues winger Gael Kakuta has joined Lazio.
The 22-year-old, who has made just one Premier League start for Chelsea, moves to Serie A on loan until the end of the season.
He embarks on a fifth loan spell away from Stamford Bridge having previously spent time at Fulham and Bolton Wanderers, and with Dijon in France, before his time with Vitesse Arnhem in Holland.
Lazio are 10th in the Serie A table ahead of Sunday's match away to Chievo.