. Author manuscript; available in PMC: 2015 Aug 23.
Published in final edited form as: Nat Struct Mol Biol. 2015 Jun;22(6):433–434. doi: 10.1038/nsmb.3041
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/.