Early Human Leukocyte Antigen (HLA) Matched Sibling Hematopoietic Stem Cell Transplantation

This study aims to enroll 58 pre-adolescent (\<13 years) pediatric participants with sickle cell disease (SCD) who have a pre-adolescent sibling bone marrow donor. All participants will go through a pre-transplant evaluation to find out if there are health problems that will keep them from being able to receive the transplant. It usually takes 2 to 3 months to complete the pre-transplant evaluation and make the arrangements for the transplant. Once they are found to be eligible for transplant, participants will be admitted to the hospital and will start transplant conditioning. Conditioning is the chemotherapy and other medicines given to prepare them to receive donor cells. It prevents the immune system from rejecting donor cells. Conditioning will start 21 days before transplant. Once they complete conditioning, participants will receive the bone marrow transplant. After the transplant, participants will stay in the hospital for 4-6 weeks. After they leave the hospital, participants will be followed closely in the clinic. Outpatient treatment and frequent clinic visits usually last 6 to 12 months. Routine medical care includes at least a yearly examination for many years after transplant by doctors and nurses familiar with sickle cell disease and transplant. The researchers will collect and study information about participants for 2 years after transplant.