Nature Structural & Molecular Biology 13, 250–255 (2006); published online 5 February 2006; corrected after print 18 April 2006

In the version of this article initially published, Figure 2 contained an error. Figure 2b was inadvertently duplicated from Figure 2a during preparation of the final version. The correct Figure 2b is shown here. The error has been corrected in the PDF and the HTML versions of this article.