Key Inclusion Criteria:

   - Diagnosis of transfusion-dependent β-thalassemia (TDT) as defined by:

      1. Documented homozygous β-thalassemia or compound heterozygous β-thalassemia
      including β-thalassemia/hemoglobin E (HbE). Subjects can be enrolled based on
      historical data, but a confirmation of the genotype using the study central
      laboratory will be required before busulfan conditioning.

      2. History of at least 100 mL/kg/year or ≥10 units/year of packed RBC transfusions
      in the prior 2 years before signing the consent or the last rescreening for
      patients going through re-screening.

   - Eligible for autologous stem cell transplant as per investigator's judgment.

Key Exclusion Criteria:

   - A willing and healthy 10/10 Human Leukocyte Antigen (HLA)-matched related donor is
   available per investigator's judgement.

   - Prior allo-HSCT.

   - Subjects with associated α-thalassemia and >1 alpha deletion or alpha multiplications.

   - Subjects with sickle cell beta thalassemia variant.

   - Clinically significant and active bacterial, viral, fungal, or parasitic infection as
   determined by the investigator.

   - White blood cell (WBC) count <3 × 10^9/L or platelet count <50 × 10^9/L not related to
   hypersplenism.

Other protocol defined Inclusion/Exclusion criteria may apply.