This is a single-dose, open-label study in pediatric participants with TDT. The study will evaluate the safety and efficacy of autologous CRISPR-Cas9 modified CD34+ human hematopoietic stem and progenitor cells (hHSPCs) (CTX001).

Key Inclusion Criteria:

• Diagnosis of TDT as defined by:

  • Documented homozygous 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 mL/kilograms (kg)/year of packed RBC transfusions in the prior 24 months before signing of consent (or the last rescreening for patients going through repeat screening) or, for participants initiating transfusion therapy <24 months before signing of consent, requirement for packed RBC transfusion at least every 3 to 4 weeks for ≥6 months

• 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 hematopoietic stem cell transplant (HSCT)
• Participants with associated α-thalassemia and >1 alpha deletion, or alpha multiplications
• Participants with sickle cell β-thalassemia variant
• Clinically significant and active bacterial, viral, fungal, or parasitic infection as determined by the investigator

Other protocol defined Inclusion/Exclusion criteria may apply.

Details on Vertex data sharing criteria and process for requesting access can be found at: https://www.vrtx.com/independent-research/clinical-trial-data-sharing