1. Department of Immunology, Faculty of Medicine, University of Tokyo, Hongo 7‐3‐1, Bunkyo‐ku, Tokyo 113, Japan
2. Institute for Molecular and Cellular Biology, Osaka University, Yamadaoka 1‐3, Suita‐shi, Osaka 565, Japan
3. Bio Signal Pathway Project, Kanagawa Academy of Science and Technology, and Meiji Institute of Health Science, Meiji Milk Products Co. Ltd, Naruda 540, Odawara‐shi, Kanagawa 250, Japan
4. Amgen Institute, 500 Sherbourne Street, Toronto, Ontario, Canada M4X 1K9