1. Structural Biology Laboratory and Gene Expression Laboratory, Salk Institute for Biology Studies, 10010 North Torrey Pines Road, La Jolla, California 92037, Department of Chemistry and Biochemistry and Divison of Biological Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92037, and Center for Regenerative Medicine in Barcelona, Dr. Aiguader, 88, 08003 Barcelona, Spain