1. Center for Blood Research, Harvard Medical School, Boston, MA 02115, U.S.A.
2. Division of Immunology, Children's Hospital, Harvard Medical School, Boston, MA 02115, U.S.A.
3. Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115, U.S.A.
4. Department of Pediatrics, Harvard Medical School, Boston, MA 02115, U.S.A.