Inclusion Criteria: 1. Male or female subjects age 2-40 years inclusive 2. Confirmed diagnosis of sickle cell disease with globin typing (e.g. HbSS, HbSC, HbS/β0-thalassemia or others) 3. Performance status \>70% (Karnofsky for subjects \>16 years of age and Lansky for subjects \<16 years of age) 4. At least one of the following indicators of disease severity as defined in the protocol - Vaso-occlusive pain crisis, Acute chest syndrome, Recurrent priapism, prior stroke, receive chronic transfusions, Red cell alloimmunization 5. Subjects, who have failed, not tolerated or refused hydroxyurea therapy. Exclusion Criteria: 1. Available matched related donor for HSCT 2. Clinically significant active infection 3. Seropositive for HIV or HTLV 4. Active known malignancy, myelodysplasia, abnormal cytogenetics or immunodeficiency 5. Prior HSCT or gene therapy 6. Known hepatic cirrhosis, bridging hepatic fibrosis or active hepatitis 7. Protocol defined iron overload 8. Cerebrovascular procedure within one year, including pial synangiosis for Moyamoya 9. Severe or progressive arteriopathy or cerebrovascular disease, including Moyamoya Other protocol defined inclusion/exclusion criteria may apply