Targeted Busulfan, Fludarabine Conditioning Regimen for Hematopoietic Stem Cell Transplantation in GRD

Inclusion Criteria: 1. Patients who are diagnosed as genetic rare disease. 2. Patients who need hematopoietic stem cell transplantation 3. Age: up to 21 years 4. Performance status: ECOG 0-2. 5. Patients must be free of significant functional deficits in major organs, but the following eligibility criteria may be modified in individual cases. 1. Heart: a shortening fraction \> 30% and ejection fraction \> 45%. 2. Liver: total bilirubin \< 2 × upper limit of normal; ALT \< 3 × upper limit of normal. 3. Kidney: creatinine \<2 × normal or a creatinine clearance (GFR) \> 60 ml/min/1.73m2. 6. Patients must lack any active viral infections or active fungal infection. 7. Appropriate donor is available: Matched in 6/6 of A, B, DR loci. 8. Patients (or one of parents if patients age \< 19) should sign informed consent. Exclusion Criteria: 1. Pregnant or nursing women. 2. Malignant or nonmalignant illness that is uncontrolled or whose control may be jeopardized by complications of study therapy. 3. Psychiatric disorder that would preclude compliance.