1. Howard Hughes Medical Institute;
2. Structural Biology Program, Department of Physiology and Biophysics, Mount Sinai School of Medicine, New York, NY 10029, USA.
3. Department of Biology, University of California, San Diego, La Jolla, CA 92093, USA.
4. Department of Endocrinology and Metabolism;
5. Transgenic Research Unit; and Department of Medicine, School of Medicine, University of California, San Diego, La Jolla, CA 92093, USA.