Nature Cell Biol. 10, 994–1002 (2008); published online 20 July 2008; corrected after print, 27 April 2010

In the version of this letter initially published online, Fig. 1c (middle panel), Fig. 3a (bottom panels) and Fig. 3c (bottom panel) were incorrectly labelled. The corrected labelling is shown below. These errors have been corrected in both the HTML and PDF versions of the letter.