Minimizing Toxicity in HLA-identical Sibling Donor Transplantation for Children With Sickle Cell Disease

Inclusion criteria: Patients with genotypes hemoglobin SS and Sβ0 thalassemia must have at least one of the following: * History of an abnormal transcranial Doppler measurement defined as TCD velocity ≥200 cm/sec by the non-imaging technique (or ≥185 cm/sec by the imaging technique) measured at a minimum of two separate occasions. * History of cerebral infarction on brain MRI (overt stroke, or silent stroke if ≥3 mm in one dimension, visible in two planes on fluid-attenuated inversion recovery T2-weighted images). * History of two or more episodes of acute chest syndrome (ACS) in lifetime. * History of three or more SCD pain events requiring treatment with an opiate or IV pain medication (inpatient or outpatient) in lifetime. * History of any hospitalization for SCD pain or ACS while receiving hydroxyurea treatment. * History of two or more episodes of priapism (erection lasting ≥4 hours or requiring emergent medical care). * Administration of regular RBC transfusions (≥8 transfusions in the previous 12 months). * At least two episodes of splenic sequestration requiring red blood cell transfusion or splenectomy after at least one episode of splenic sequestration. Patients with all other sickle genotypes (hemoglobin SC, Sβ+ thalassemia) must have at least one of the following: * Clinically significant neurologic event (overt stroke). * History of two or more episodes of ACS in the 2-years period preceding enrollment. * History of three or more SCD pain events requiring tre…