Nat. Struct. Mol. Biol. 19, 1023–1030 (2012); published online 16 September 2012; corrected after print 27 December 2012

In the version of this article initially published, Simone Meidhof's affiliations should have included Spemann Graduate School of Biology and Medicine (SGBM), Albert Ludwigs University Freiburg, Freiburg, Germany and the Faculty of Biology, Albert Ludwigs University Freiburg, Freiburg, Germany. Acknowledgement of the Excellence Initiative of the German Research Foundation (GSC-4, Spemann Graduate School) was omitted. The errors have been corrected in the HTML and PDF versions of the article.