Nature Biotechnology 25, 303–305 (2007); published online 7 March 2007; corrected after print 1 June 2007.

In the version of this article initially published, reference 1 is incorrect. The correct reference is Gavin, A.L. et al. Science 314, 1936–1938 (2006). The incorrect reference has been replaced in the HTML and PDF versions of the article.