4. Suitable Donor: To undergo transplantation on this study, participants must have an adult first degree relative who shares at least 1 human leukocyte antigen (HLA) haplotype with the participant, does not have SCD or other hemoglobinopathy, and is in good health; if these criteria are met, they will be allowed to serve as donors. Relatives with sickle cell trait are not excluded as donors. When more than 1 donor is available, the donor with the fewest HLA allele mismatches will be chosen, unless the patient had donor anti-HLA antibodies or there was a medical reason to exclude the donor. If donor anti-HLA antibodies are detected, the next best related match will be chosen. Umbilical cord blood or peripheral blood stem cell donors will not be accepted.