1. Departments of Biological Chemistry and Molecular Pharmacology, Medicine, and Pediatrics, Harvard Medical School, Hematology Division, Brigham and Women's Hospital, Division of Hematology−Oncology, Children's Hospital, Boston, Massachusetts 02115, Department of Biomedical Research, St. Elizabeth's Medical Center of Boston, Tufts University Medical School, Boston, Massachusetts 02135, and Universitäts-Kinderklinik, D-37075 Göttingen, FRG