Nat. Biotechnol. 6, 549–555 (2009); published online 24 May 2009; corrected after print 8 July 2009

In the version of this article initially published, Figure 2f is not referenced in the figure legend and is referenced as Figure 2e in the main text. Also, on p.5, right col., para. 1, line 8, miR-21 should be miR-122. The errors have been corrected in the HTML and PDF versions of the article.