• Patients with sickle cell anemia (Hb SS, Sβ0 thalassemia or severe SC) who are 18 - 50 years of age inclusive AND who have 1 or more of the following:

‣ Clinically significant neurologic event (stroke) or any neurological deficit lasting at least 24 hours. Stroke will be defined as a clinically significant neurologic event that is accompanied by an infarct on cerebral MRI or cerebral arteriopathy requiring chronic transfusion therapy.

⁃ History of two or more episodes of ACS in the 2-year period preceding enrollment despite supportive care measures (i.e. asthma therapy and/or hydroxyurea).

⁃ History of three or more severe vaso-occlusive pain crises per year in the 2-year period preceding enrollment despite the institution of supportive care measures (i.e. a pain management plan and/or treatment with hydroxyurea).

⁃ Administration of regular red blood cell (RBC) transfusion therapy, defined as receiving 8 or more transfusions per year for 1 year or more to prevent vaso-occlusive clinical complications (i.e. pain, stroke, and ACS)

⁃ An echocardiographic finding of tricuspid valve regurgitant jet (TRJ) velocity \> or equal to 2.7 m/sec or pulmonary hypertension diagnosed by right heart catheterization.

⁃ Chronic kidney disease including patients on hemo-dialysis

⁃ Recurrent tricorporal priapism defined as at least 2 episodes of an erection last ≥4 hours involving the corpus cavernosa and corpus spongiosa.

⁃ Recipient cannot be pregnant or lactating.

• Adequate organ functions as defined as:

‣ Eastern Cooperative Group (ECOG) performance status of 2 or better

⁃ Cardiac function: left ventricular ejection fraction (LVEF) of 40% or greater

⁃ Pulmonary Function: Pulse oximetry with a baseline oxygen saturation of 85% or greater and corrected diffusing capacity of the Lungs for carbon monoxide (DLCO) of 35% or greater

⁃ Hepatic Function: Serum conjugated (direct) bilirubin less than 3x upper limit of normal for age as per local laboratory, alanine aminotransferase (ALT) and aspartate transaminase (AST) less than 5 x upper limit of normal as per local laboratory. Patients whose hyperbilirubinemia is the result of hyperhaemolysis, or a sever drop in hemoglobin post blood transfusion are not excluded.

⁃ Absence of liver cirrhosis, bridging fibrosis and active hepatitis as documented by liver biopsy for patients with evidence of iron overload by serum ferritin or MRI. The histological grading and scale described by Ishak and colleagues (1995) will be used.

• Patient must have a matched-or mismatched unrelated donor or mismatched related family donor.

‣ For HLA-matching we will assess 12 HLA-antigens (HLA-A, B, C, DRB1, DQB1 and DPB1).

⁃ Fully matched unrelated transplanted are defined as matched at 12/12 HLA-alleles. We will include up to 7/8 (HLA-A, B, C, and DRB1) matched unrelated donors.

⁃ One haplotype-mismatched related donors will be included.