Study Overview: This study is focusing on babies with Severe Combined Immunodeficiency (SCID), a condition where babies have very weak immune systems. Researchers are testing if low doses of chemotherapy drugs can help these babies build a stronger immune system with fewer side effects after they receive a bone marrow transplant. The study uses two different doses of a chemotherapy drug called busulfan. Babies will get their transplants early in life, using donor cells that are not a perfect match but are safe to use. After the transplant, babies will be monitored for 3 years to see how well their immune systems recover.

Key Points:

• Babies will be followed for 3 years after the transplant to monitor immune recovery.
• The study is only for babies using unrelated or partially matched donor cells.
• Vaccinations will be given and tested 9-18 months post-transplant to check immune response.

Please consult with healthcare providers to understand if this study is suitable for your child and discuss any potential risks or benefits.