Nature Genetics 38, 1386–1396 (2006); published online 12 November 2006; corrected after print 27 February 2008

In the version of this article initially published, the horizontal dashed lines representing the threshold value in the panels in row b of Figures 2 and 4 were incorrectly placed. The errors have been corrected in the HTML and PDF versions of this article. See below for the corrected version of the figures.