. Author manuscript; available in PMC: 2013 Nov 1.
Published in final edited form as: Nat Struct Mol Biol. 2013 Mar 31;20(5):574–581. doi: 10.1038/nsmb.2535
Users may view, print, copy, download and text and data- mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#terms