Nat. Struct. Mol. Biol. 14, 295–300 (2007); published online 1 April 2007; corrected after print 1 April 2007

In the version of this article initially published, three references were missing from the reference list. The corrected reference list is shown below, and references have been renumbered accordingly. The error has been corrected in the HTML and PDF versions of the article.