1. Center of Excellence in Sickle Cell Disease, Department of Medicine, Boston University School of Medicine, Boston, MA;
2. Department of Pediatrics, Johns Hopkins University School of Medicine, Baltimore, MD;
3. Department of Biostatistics, Boston University School of Public Health, Boston, MA; and
4. Sickle Cell Disease Research Center and Department of Pediatrics, College of Medicine, King Saud University, Riyadh, Saudi Arabia