Nat. Cell Biol. 12, 507–512 (2010); published online 18 April 2010; corrected after print, 9 June 2010

In the version of this letter initially published online and in print, Fig. 1b (inset graph) and Fig. 5e were incorrectly labelled. The correct labelling is shown below. These errors have been corrected in both the HTML and PDF versions of the letter.