This is a single-arm, open-label, multi-site, single-dose Phase 1/2/3 study in participans with transfusion-dependent β-thalassemia (TDT). The study will evaluate the safety and efficacy of autologous CRISPR-Cas9 Modified CD34+ Human Hematopoietic Stem and Progenitor Cells (hHSPCs) using CTX001.

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). 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
  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
• Participants with associated α-thalassemia and >1 alpha deletion or alpha multiplications
• Participants 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.

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