Nat. Struct. Mol. Biol. 19, 1214–1215 (2012); published online 5 December 2012; corrected after print 20 December 2012

In the version of this article initially published, the first author's name in ref. 3 was given as 'Brein'; the correct spelling is 'Brien'. The error has been corrected in the HTML and PDF versions of the article.