Nat. Cell Biol. 13, 497–505 (2011); published online 3 May 2011; corrected after print 28 April 2011

In the version of this review initially published in print, reference 105 was incorrect. This reference has now been removed, and the remaining references have been renumbered. Sentences referring to this reference have been removed, and the text has been reworded accordingly. This error has been corrected online in both the HTML and PDF versions of the review.