. Author manuscript; available in PMC: 2020 Apr 15.
Published in final edited form as: Curr Opin Struct Biol. 2019 Nov 18;61:17–24. doi: 10.1016/j.sbi.2019.10.001
This is an open access article under the CC BY-NC-ND license (http://creative-commons.org/licenses/by-nc-nd/4.0/).