The major goal of this study is to determine the risks and benefits of stem cell transplants in combination with a newer, less toxic conditioning chemotherapy treatment in patients with severe sickle cell disease (SCD) or sickle hemoglobin variants (hemoglobin SC or hemoglobin SB0/+), or homozygous b0/+ thalassemia or severe B0/+ thalassemia variants. Participation in this project will be for one year, with follow up evaluations done every 6 months thereafter for 10 years or until participants are 18 years old.