Additional Study Information: This study is being done to learn about the safety and efficacy of CTX001 (the "Study Product") to treat -Thalassemia. CTX001 is "investigational", which means that the Study Product is considered experimental and is not approved by the Food and Drug Administration (FDA), European Medicines Agency (EMA) or any other regulatory (government) authority and is still being tested to see if it is safe and if it works. The procedures in this study include a screening period (up to 8 weeks), blood stem cell collection and study drug manufacture (2-3 months), chemotherapy conditioning and study product infusion (1 month) and a follow up period of 2 years.