Nat. Struct. Mol. Biol. 19, 90–97 (2012); published online 18 December 2011; corrected after print 9 January 2012

In the version of this article initially published, information in Table 1 was inaccurate. “Newly described” should have been “novel” and “Argonaute protein domain” should have read “Argonaute protein.” The errors have been corrected in the HTML and PDF versions of the article.