1. Department of Immunology and Microbial Sciences and Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037
2. Basic Sciences Division, Fox Chase Cancer Center, Philadelphia, PA 19111
3. Cell Biology and Physiology Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892
4. Department of Cell Biology, Harvard Medical School, Boston, MA 02115