Nature Genetics 39, 1488–1493 (2007); published online 11 November 2007; corrected after print 23 January 2008

In the version of this article initially published, the affiliation for C S Raman was incorrect. Dr. Raman is affiliated with the Department of Biochemistry and Molecular Biology at the University of Texas Health Science Center, and not with the Structural Biology Center. The error has been corrected in the PDF version of the article.