1. Integrated Program in Cellular, Molecular, and Biophysical Studies, Howard Hughes Medical Institute, Columbia University, College of Physicians and Surgeons, 701 West 168th Street, Room 720, New York, NY 10032, USA.
2. Department of Biochemistry and Molecular Biophysics, Howard Hughes Medical Institute, Columbia University, College of Physicians and Surgeons, 701 West 168th Street, Room 720, New York, NY 10032, USA.