Nat. Genet. 46, 398–404 (2014); published online 16 March 2014; corrected after print 7 May 2014

In the version of this article initially published, there were errors in the layout of Figure 2 that resulted in truncations of the right portion of Figure 2b and the bottom portion of Figure 2c. These errors have been corrected in the HTML and PDF versions of the article.