Nat. Struct. Mol. Biol. 14, 997–1007 (2007); published online 5 November 2007; corrected after print 5 November 2007

In the print version of this article, the reference to the Review by Shilatifard and co-workers is incorrect. On page 1002, it should be referred to as 'Bhaumik et al.68' and on page 1006, the reference citation should read: