Department of Epidemiology, Vitalant Research Institute, San Francisco, California, USA. Department of Pediatrics, Division of Hematology/Oncology, UCSF Benioff Children's Hospital Oakland, Oakland, California, USA. Rodeghier Consultants, Chicago, Illinois, USA. Department of Pediatrics, Division of Hematology/Oncology, Vanderbilt-Meharry Center for Excellence in Sickle Cell Disease, Vanderbilt University Medical Center, Nashville, Tennessee, USA.