Nat. Struct. Mol. Biol. 14, 45–53 (2007); published online 10 December 2006; corrected after print 15 June 2007

In the version of this article initially published, the crystallization conditions were incorrectly reported. The correct conditions are as follows: 95 mM sodium citrate (pH 5.6), 19% (v/v) isopropanol, 19% (w/v) PEG 4,000 and 5% (v/v) glycerol. The error has been corrected in the HTML and PDF versions of the article. The authors apologize for any inconvenience this may have caused.