Nat. Genet. 43, 48–50 (2011); published online 28 November 2010; corrected after print 24 February 2011

In the version of this article initially published, the third and fourth column headings in Table 2 were mislabeled and the abbreviations in the footnotes of Table 2 were inadvertently duplicated. The correct heading for the third column is “T” and the correct heading for the fourth column is “NT”. These errors have been corrected in the HTML and PDF versions of the article.