Adding Azathioprine/Hydroxyurea Preconditioning to Alemtuzumab/TBI to Reduce Risk of Graft Failure in MSD HSCT in Adult SCD Patients

Inclusion Criteria: * SCD patients with an HLA-identical matched sibling donor eligible for allogeneic stem cell transplantation. * Age 16 - 60 years * Good performance status (ECOG 0 or 1; Karnofsky and Lansky 70-100) * Patients and donors (MSD) must be able to sign consent forms for receiving and donating hematopoietic stem cells respectively. The sibling donor should be willing to donate. * Patients must be geographically accessible and willing to participate in all stages of treatment. * Eligible diagnoses: Patients with sickle cell disease such as sickle cell anemia (Hb SS), Hb/Sβ0-thalassemia, Hb/Sβ+-thalassemia, HbSC disease, HbSE disease, HbSD disease and Hemoglobin SO- Arab disease. Exclusion Criteria: * Poor performance status (ECOG\>1). * Poor cardiac function: left ventricular ejection fraction\<35%. * Poor pulmonary function: FEV1 and FVC\<40% predicted. * Poor liver function: direct bilirubin \>3.1 mg/dl * HIV-positive * Women of childbearing potential who currently are pregnant (Beta-HCG+) or who are not practicing adequate contraception. * Patients who have any debilitating medical or psychiatric illness that would preclude their giving informed consent or their receiving optimal treatment and follow-up. However, patients with history of stroke and significant cognitive deficit, that would preclude giving informed consent or assent will not be excluded, if they have a family member or significant other with Power of Attorney to also consent of their behalf.