Key Inclusion Criteria:  Subjects and/or legal representative fully understand and voluntarily sign informed consent forms. Diagnosis of transfusion dependent β-thalassemia (β-TDT) as defined by protocol. Be not appropriate to receive conventional allogeneic hematopoietic stem cell transplantation. Lansky/Karnofsky score ≥ 70%. Eligible for hematopoietic stem cell transplantation and conditioning with busulfan as per investigator's judgement.  Other protocol defined inclusion criteria may apply.  Key Exclusion Criteria:  Subjects with associated α-thalassemia. Subjects with any clinically significant acute or uncontrolled infections. History of uncontrolled epilepsy or other mental disorders. Previous treatment with allogeneic bone marrow transplantation or gene therapy.  Other protocol defined exclusion criteria may apply.