This is a single-arm, multi-site, single-dose, Phase 3 study in approximately 15 subjects ≥12 and ≤50 years of age with transfusion-dependent β-thalassemia (TDT), who have a β0 mutation at both alleles of the β-globin (HBB) gene (i.e. β0/β0). The study will evaluate the efficacy and safety of autologous hematopoietic stem cell transplantation (HSCT) using LentiGlobin BB305 Drug Product.