1. Department of Biological Chemistry, Howard Hughes Medical Institute, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
2. Sloan-Kettering Institute, 521 Rockefeller Research Labs, 1275 York Avenue, Box 252, New York, NY 10021, USA.
3. Division of Biology 216-76, California Institute of Technology, Pasadena, CA 91125, USA.