Nat. Struct. Mol. Biol. 18, 56–60 (2011); doi:10.1038/nsmb.1946; published online 12 December 2010; corrected after print 9 August 2011 and 10 October 2012

In the version of this article initially published, support from the Oklahoma Center for the Advancement of Science and Technology to R.J.P. was not acknowledged. The error has been corrected in the PDF and HTML versions of this article.