This study involves a partial stem cell transplant for patients with **severe sickle cell disease** or **beta-thalassemia**. **Stem cells** are special cells that can become different types of cells in the body. The transplant is non-ablative, which means it's less intense and doesn't destroy all the existing cells. The goal is to have more **donor cells** stay in the patient's body over time. The study will evaluate how well the transplant works, focusing on the survival of the donor cells and the health of the patient. Patients will receive a slightly higher dose of **total body irradiation (TBI)**, which is a type of radiation therapy, to help the donor cells take hold more effectively. Medications like **sirolimus** and **abatacept** are used to prevent the body from rejecting the new cells. **Key Points:** - **Study Duration:** Long-term follow-up with regular check-ins for up to three years. - **Eligibility:** Patients aged 4-65 with severe sickle cell or beta-thalassemia, and matched donors. - **Risks:** Potential for graft failure and monitoring for complications like infections and GVHD (Graft-Versus-Host Disease). Patients are advised to discuss participation with their doctors to understand the benefits and risks.