A Study of Immune Suppression Treatment for People With Sickle Cell Disease or β-Thalassemia Who Are Going to Receive an Allogeneic Hematopoietic Cell Transplantation (HCT)

Inclusion Criteria: * Age ≥ 2 and ≤ 50 years * Suitable haploidentical donor. * Performance score ≥ 70% by Karnofsky Performance Scale or 0 to 1 by ECOG (age \> 16 years), or Lansky Play-Performance Scale ≥ 70% (age ≤ 16 years). * Adequate major organ system function as demonstrated by: * For patients ≥ 18 years of age: * eGFR ≥ 50 mL/min by Cockcroft-Gault formula Formula: ((140 - Age) x Weight (kg)) / (72 x Serum Creatinine (mg/dL) Female Adjustment: Multiply result by 0.85 * For patients \< 18 years of age: * Serum creatinine clearance: glomerular filtration rate \[GFR\]) must be \>50 mL/min/1.73 m2 as calculated by the Schwartz formula * Conjugated (direct) bilirubin less than 3x upper limit of normal. * ALT or AST ≤ 3 times institutional upper limit of normal. * Left ventricular ejection fraction ≥ 50%. * Diffusing capacity for carbon monoxide (DLCO) ≥ 50% predicted, corrected for hemoglobin. For children \< 7 years of age who are unable to perform PFT, oxygen saturation \> 92% on room air by pulse oximetry. * For SCD patients: HbSS, HbSC, HbS/β° with one or more of the following complications: * Acute chest syndrome: 2 or more episodes in the 2 years preceding enrollment * Vaso-occlusive episodes: 3 or more episodes in the 2 years preceding enrollment * Recurrent priapism: 2 or more episodes in the 2 years preceding enrollment * History of osteomyelitis or osteonecrosis * Cerebrovascular disease: * Imaging evidence of prior overt or silent stroke * …