1. The Jackson Laboratory, Bar Harbor, Maine 04609
2. Department of Embryology, Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978
3. Department of Immunology and Pathology, Kimron Veterinary Institute, Bet-Dagan 50250, Israel
4. Laboratory of Immunopathology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892