A Phase 3b Study to Evaluate Efficacy and Safety of a Single Dose of Autologous CRISPR Cas9 Modified CD34+ Human Hematopoietic Stem and Progenitor Cells (CTX001) in Subjects With Transfusion-Dependent β-Thalassemia or Severe Sickle Cell Disease

Key Inclusion Criteria: * Participants with TDT and SCD: * Eligible for autologous stem cell transplant as per investigator's judgment. * Participants with TDT: * Diagnosis of TDT as defined by: * Documented homozygous β-thalassemia or compound heterozygous β-thalassemia including β-thalassemia/hemoglobin E (HbE). Participants can be enrolled based on historical data, but a confirmation of the genotype using the study central laboratory will be required before busulfan conditioning * History of at least 100 milliliter (mL)/kilograms (kg)/year or 10 units/year of packed red blood cells (RBC) t…