Nature Genetics 38, 694–699 (2006); published online 20 April 2006; corrected after print 9 June 2006.

In the version of this article initially published, the order of the first three authors was incorrect. The correct order should be Bas Tolhuis, Elzo de Wit, Inhua Muijrers. This error has been corrected in the PDF version of the article.