1. Department of Molecular Biology, Research Institute of Molecular and Cellular Biology, 901 California Avenue, Palo Alto, CA 94304
2. Department of Immunology, DNAX Research Institute of Molecular and Cellular Biology, 901 California Avenue, Palo Alto, CA 94304
3. Department of Molecular and Developmental Biology, The Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108, Japan
4. Department of Molecular Biology, DNAX Research Institute of Molecular and Cellular Biology, 901 California Avenue, Palo Alto, CA 94304, and Department of Molecular and Developmental Biology, The Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108, Japan