1. From the Division of Hematology and Oncology, Children's Hospital Medical Center and Sidney Farber Cancer Center, and the Department of Pediatrics, Harvard Medical School, Boston, MA, the Division of Hematology, Children's Hospital of Philadelphia, Philadelphia, PA, and the Departments of Genetics and Obstetrics and Gynecology, Yale–New Haven Hospital, New Haven, CT (address reprint requests to Dr. Nathan at the Division of Hematology and Oncology, Children's Hospital Medical Center, 300 Longwood Ave.,...