. Author manuscript; available in PMC: 2019 Sep 4.
Published in final edited form as: Nat Struct Mol Biol. 2019 Mar 4;26(3):204–212. doi: 10.1038/s41594-019-0191-4
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