Howard Hughes Medical Institute, University of California San Diego, La Jolla, California, United States of America. xcheng@mccammon.ucsd.edu|Howard Hughes Medical Institute, University of California San Diego, La Jolla, California, United States of America. xcheng@mccammon.ucsd.edu|Howard Hughes Medical Institute, University of California San Diego, La Jolla, California, United States of America. xcheng@mccammon.ucsd.edu|Howard Hughes Medical Institute, University of California San Diego, La Jolla, California, United States of America. xcheng@mccammon.ucsd.edu|Howard Hughes Medical Institute, University of California San Diego, La Jolla, California, United States of America. xcheng@mccammon.ucsd.edu