Nat. Biotechnol. 24, 1420–1428 (2006); published online 22 October 2006; corrected after print 7 February 2007.

In the version of the article initially published, in the Author Contributions, the initials of the last author responsible for the virus challenge and related experiments are H.-W.W. and not H.-W.N., as originally indicated. In Figure 2e, the amiR-P69159 line is line number 1 and not line number 11, as originally indicated. The error has been corrected in the PDF version of the article.