Nat. Biotechnol. 24, 963–970 (2006); published online 9 August 2006; corrected after print 10 October 2006

In the version of this article initially published, errors appeared in equations 1–6 and in the expression at the end of the first line after equation 4. Some errors appeared in the print only and others in the online pdf only. The correct equations appear below and have been corrected in the HTML and PDF versions of the article.