Nat. Chem. Biol. 10, 927–929 (2014); published online 21 September 2014; corrected after print 19 August 2015

In the version of this Brief Communication initially published, the email address for corresponding author, Chao Xu, was incorrect. It should be listed as chaor.xu@utoronto.edu. This error has been corrected in the PDF and HTML versions of the article.