1. Department of Pathology, University of California-San Diego, La Jolla 92093.
2. Department of Medicine, University of California-San Diego, La Jolla, CA 92093
3. School of Medicine, University of Utah, Salt Lake City, UT 84132
4. Eukaryotic Regulatory Biology Program and the Howard Hughes Medical Institute, University of California-San Diego, La Jolla, CA 92093.