Nat. Struct. Mol. Biol. 18, 1211–1217 (2011); published online 7 October 2011; corrected after print 8 November 2011

In the version of this article initially published, several acknowledgments were missing. The end of the section should read as follows: “...N.S., who was also supported by a Howard Hughes Medical Institute (HHMI) International Scholarship. Postdoctoral support was from the Terry Fox Foundation of the Canadian Cancer Society to M.R.F. T.F.D. was supported through a CIHR grant. M.K.C. was supported by the Foundation for Polish Science International PhD Projects. F.F. was supported by a Boehringer Ingelheim Fonds PhD fellowship.” The error has been corrected in the HTML and PDF versions of the article.