Inclusion Criteria: 1. Diagnosis of SCID-X1 based on: * A proven mutation in the common interleukin-2 receptor gamma chain gene as defined by direct sequencing of patient DNA. * T-cell immune deficiency defined as one or more of the following: CD3+ autologous T cells \< 300/ul, or less than 50% of normal value for in vitro mitogen stimulation, or absent proliferation in vitro to antigen stimulation. 2. No available HLA identical related donor. 3. With severe infections, including but not limited to: pneumonitis; protracted diarrhea requiring total parenteral nutrition; infection with herpes viruses or adenovirus; disseminated BCG infection. 4. No cytogenetic abnormalities (medullary karyotype) and no detection of main rearrangements associated with acute leukemia. 5. No prior allogeneic stem cell transplantation. 6. Life expectancy ≥ 3 months. 7. Documented to be negative for HIV infection. 8. Written, informed consent obtained prior to any study-specific procedures. Exclusion Criteria: 1. No available molecular diagnosis confirming SCID-X1. 2. Existence of an available HLA-identical related donor. 3. Diagnosis of active malignant disease other than EBV-associated lymphoproliferative disease. 4. Current treatment with any chemotherapeutic agent (becomes eligible if not on treatment for at least 1 month). 5. Patients with evidence of infection with HIV-1 or 2. 6. Presence of a medical condition indicating that survival will be less than 4 weeks such as the requirement for mechanical ventilation, severe failure of a major organ system, or evidence of a serious, progressive infection that is refractory to medical treatment. 7. Current treatment with any immunosuppressive agent, excluding corticosteroids. 8. Patients, in the opinion of investigators, may not be eligible or not able to comply with the study.