. Author manuscript; available in PMC: 2020 Aug 17.
Published in final edited form as: Nat Struct Mol Biol. 2020 Feb 17;27(3):229–232. doi: 10.1038/s41594-020-0383-y
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