This study checks on people who've already received a special treatment called CTX001. CTX001 is a type of gene therapy, which is a treatment that changes a person's genes to help treat diseases like transfusion-dependent β-thalassemia (TDT) or severe sickle cell disease (SCD). This study is to make sure the treatment continues to be safe and effective over a long time. People who want to join must have already had CTX001 in a previous study. They or their guardians have to agree by signing a consent form. There are no specific reasons that would stop someone from participating.

• Length of Study: This is a long-term study, so it will go on for several years.
• Visits Needed: Participants might need to visit the study site multiple times.
• Risks: The study will watch for any long-term effects or risks of the treatment.