This study focuses on young patients (under 21) with non-cancerous diseases like sickle cell disease, immune disorders, or metabolic problems. A bone marrow transplant (BMT) is a procedure to replace damaged or destroyed bone marrow with healthy bone marrow stem cells. In this study, patients will receive a reduced intensity conditioning regimen, which uses lower doses of chemotherapy drugs like hydroxyurea and fludarabine, making the procedure less intense.

After the conditioning, participants will undergo a familial HLA-mismatched bone marrow transplant. HLA mismatching means the donor's tissue type doesn't fully match the recipient's, but a family member can still donate. The study aims to ensure the safety of the procedure and to monitor the integration (engraftment) of donor cells 100 days and 1 year after the transplant.

• Participants must be under 21 with specific non-cancerous conditions.
• The study involves a less intense treatment and family donor participation.
• Safety and successful cell integration are the primary goals.