Nat. Chem. Biol. 9, 756–757 (2013); published online 20 October 2013; corrected after print 11 December 2013

In the version of this article initially published, the citation to reference 4 incorrectly referred to “Castro-Roa, K.”. The correct author's name is “Castro-Roa, D.”. The error has been corrected in the HTML and PDF versions of the article.