1. Department of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA 98104.
2. Freeman Laboratory of Cancer Cell Biology, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724.
3. Department of Physiology, University of California, San Francisco, CA 94143.
4. Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka City 812, Japan.