Nat. Struct. Mol. Biol. 17, 1024–1026 (2010); published online 18 July 2010; corrected after print 25 August 2010

In the version of this article initially published, “2′-O-methyl group at the 10th nucleotide” should have read “2′-O-methyl groups at the 9th and 10th nucleotides.” The error has been corrected in the HTML and PDF versions of the article.