Nat. Cell Biol. 9, 604–611 (2007); published online 15 April 2007; retracted online 31 October 2014

Although we believe that the key finding and conclusions are still valid, recently detected image manipulation in the published figures undermines our full confidence in the integrity of the study. We therefore wish to retract this Letter. We wish to acknowledge that the co-authors from collaborating groups (M.N., H.E., T.K., C.F. and B.W.O'M.) were not involved in the figure preparation.

We deeply regret any inconvenience this retraction may cause.