A Study Evaluating the Efficacy and Safety of the LentiGlobin® BB305 Drug Product in Participants With Transfusion-Dependent β-Thalassemia

Inclusion Criteria: • Participants less than or equal to (\<=) 50 years of age at the time of consent or assent (as applicable), and able to provide written consent (adults, or legal guardians, as applicable) or assent (adolescents or children). Provided that the data monitoring committee (DMC) has approved enrolling participants younger than 5 years of age, participants younger than 5 years of age may be enrolled if they weigh a minimum of 6 kilograms (kg) and are reasonably anticipated to be able to provide at least the minimum number of cells required to initiate the manufacturing process. * Diagnosis of TDT with a history of at least 100 milliliter per kilogram per year (mL/kg/year) of pRBCs in the 2 years preceding enrollment (all participants), or be managed under standard thalassemia guidelines with \>= 8 transfusions of pRBCs per year in the 2 years preceding enrollment (participants \>=12 years). * Clinically stable and eligible to undergo HSCT. * Treated and followed for at least the past 2 years in a specialized center that maintained detailed medical records, including transfusion history. Exclusion Criteria: * Presence of a mutation characterized as other then β0 (e.g., β+, βE, βC) on at least one β-globin gene (HBB) allele. * Positive for presence of human immunodeficiency virus type 1 or 2 (HIV-1 and HIV-2), hepatitis B virus (HBV), or hepatitis C (HCV). * A white blood cell (WBC) count less than (\<) 3×10\^9/liter (L), and/or platelet count \<100×10\^9/L n…