. Author manuscript; available in PMC: 2015 Jul 13.
Published in final edited form as: Curr Opin Struct Biol. 2015 Feb 21;30:89–99. doi: 10.1016/j.sbi.2015.02.001
© 2015 The Authors.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).