Nat. Genet. 33, 102–106 (2003); published online 16 December 2002; corrected after print 27 August 2015
In the version of this article initially published, Figure 2 was inappropriately enhanced. The original data have been restored to the figure. The authors state that all the display items in this publication are previously unpublished work and accurate representations of the experiments described. The error has been corrected in the HTML and PDF versions of the article.