The second component will include patients who have a related or an unrelated human leukocyte antigen (HLA) matched donor. Up to 15 additional patients will be transplanted in this component of the study which will evaluate the safety and feasibility of unrelated donor hematopoietic cell transplantation (HCT) in adults with SCD. Data related to study endpoints for 1 year after transplantation will be collected; however, participating centers will be encouraged to conduct long-term follow-up evaluations of patients according to standard institutional guidelines. The purpose of this pilot safety trial is to see if this approach is feasible and meets accrual goals lending support to the development of a subsequent full scale investigation of HCT and comparing outcomes in a transplantation cohort to a control cohort of adults eligible for, but unwilling or unable to receive HCT treated by supportive therapy with a primary endpoint of five years survival for this full scale comparative trial.