Nat. Struct. Mol. Biol. 18, 256–261 (2011); published online 27 February 2011; corrected after print 17 March 2011

In the version of this article initially published, in several instances “guanidine” should have read “guanine”, “uridine” should have read “uracil”, and “adenine” should have read adenosine; in two instances; “tetratrispolin” should have read “tristetraprolin”; and Figure 2c,d should have illustrated the structure from PDB record 1M8Y. These errors have been corrected in the HTML and PDF versions of the article.