Nat. Genet. 42, 332–337 (2010); published online 14 March 2010; corrected after print 29 March 2011
In the version of this article initially published, Kathryn Roeder's affiliation was incorrect. Her correct affiliation is the Department of Statistics, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA. The error has been corrected in the HTML and PDF versions of the article.