Inclusion Criteria: 1. Subjects voluntarily sign informed consent by themselves or their legal guardians and complete the study procedures, follow-up examination and treatment. 2. At the time of screening, subjects should be aged from 12 to 35 years old, regardless of gender. 3. History of at least 100 mL/kilograms (kg)/year of packed red blood cells (pRBC) transfusions in the prior 2 years before screening. 4. Subjects diagnosed with Hemoglobin H-Constant Spring disease (--/ααCS) with HBA2 c.427T\>C mutation. Exclusion Criteria: 1. Subject who has an available HLA-matched/well-matched HSCT donor for allogeneic hematopoietic stem cell transplantation (HSCT). 2. Prior HSCT or gene therapy. 3. History of severe hemorrhagic disease. 4. Clinically significant active bacterial, viral, fungal or parasitic infections per investigator's judgement at the time of screening.