Nat. Med. 13, 803–811 (2007); published online 1 July; corrected after print 24 July 2007

The version of this article initially published contained several typographical errors affecting figure citations, units of measure and figure legends, none of which change the scientific conclusions of the manuscript in any way. In addition, the authors incorrectly stated that they had no competing financial interests. A proper description of these competing interests, as is required by journal policy, has now been attached to the HTML version of the article, and the typographical errors have been corrected in the HTML and PDF versions.