Huilin Li1, Michael E O'Donnell2,3. 1. Structural Biology Program, Van Andel Research Institute, Grand Rapids, MI 49503. 2. Program of Biochemistry and Structural Biology, The Rockefeller University, New York, NY 10065; odonnel@rockefeller.edu. 3. Howard Hughes Medical Institute, The Rockefeller University, New York, NY 10065.