. Author manuscript; available in PMC: 2016 Oct 25.
Published in final edited form as: Nat Struct Mol Biol. 2016 Apr 25;23(6):531–539. doi: 10.1038/nsmb.3212
Users may view, print, copy, and download 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