Nature Cell Biol. 11, 988–993 (2009); published online 13 July 2009; corrected after print, 24 March 2010

In the version of this letter initially published online and in print, the labels '0 block' and '2 block' in Fig. 2a were swapped. The correct version of this figure is shown below. This error has been corrected in both the HTML and PDF versions of the article.