1. Amgen Institute, Toronto, Ontario, Canada.
2. Ontario Cancer Institute, Departments of Immunology and Medical Biophysics, University of Toronto, Toronto, Ontario M4X 1 K9, Canada.
3. Wellesley Hospital Research Institute, Toronto, Ontario M4Y 1,J3, Canada.
4. Department of Pathology, University of Toronto, Toronto, Ontario M4X 1 K9, Canada.
5. Institute for Molecular and Cellular Biology, Osaka University, Yamada-oka 1-3, Suita-shi, Osaka 565, Japan.