1. Amgen, Thousand Oaks, CA 91320.
2. Laboratory of Developmental Hematopoiesis and Laboratory of Molecular Hematology, Memorial Sloan-Kettering Cancer Center, New York, NY 10021.
3. Amgen Development, Inc., Boulder, CO 80301.
4. Institute for Clinical Immunology and Rheumatology, University of Erlangen, 8520 Erlangen, Federal Republic of Germany.