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.