Nat. Genet. 41, 365–370 (2009); published online 15 February 2009; corrected after print 9 April 2010

In the version of this article initially published, the colony formation assay image labeled “Co115.DICER1” was the incorrect image. The error has been corrected and a corrected version of Figure 4 panel e is provided in the HTML and PDF versions of the article.