Nat. Struct. Mol. Biol. 20, 769–771 (2013); published online 3 July 2013; corrected after print 12 July 2013

In the version of this article initially published, in Figure 2 the asterisks representing mutations to the PIWI domain that turn Argonaute into an active enzyme should have been colored red. The error has been corrected in the HTML and PDF versions of the article.