Nat. Cell Biol. 17, 697–705 (2015); published online 13 April 2015; corrected after print 8 June 2015

In the version of this Letter originally published, the following sentence was omitted from the Acknowledgements: 'B.L. is supported by a post-doctoral fellowship from FRM (ARF20140129055).' This has been corrected in the online versions of the Letter.