Nature Structural & Molecular Biology 14, 243–45 (2007); published online 18 February 2007; corrected after print 19 March 2007

In the version of this article initially published, a gel in Figure 2a is incorrectly labeled. The second gel down should be labeled H3-K4me3. The corrected figure is shown here. The error has been corrected in the HTML and PDF versions of this article.