X-linked severe combined immunodeficiency (XSCID) results from mutations in IL2RG, which encodes the common gamma chain (γc) shared by receptors for IL-2, 4, 7, 9, 15 and 21. XSCID is best treated with bone marrow transplantation (BMT) from an HLA-matched sibling. Patients lacking a matched sibling can benefit from a T-cell depleted haploidentical BMT, but some do not achieve adequate immune reconstitution. Ex vivo autologous hematopoietic stem cell (HSC) gene therapy may be an alternative to haploidentical BMT. In a French trial, 9 of 10 XSCID infants had immune reconstitution following ex vivo transduction of autologous HSC with a retroviral vector encoding γc. Selective development and expansion of T, NK and B cells from progenitors expressing γc was important to the success of this therapy. However, the 2 youngest patients, treated at 1 and 3 months of age, later developed T cell leukemias associated with retrovirus insertions that activated the LMO2 transcription factor. Young age at treatment might have had a role in the development of these adverse events. We have developed an XSCID gene transfer protocol as salvage treatment for older patients who have failed haploidentical BMT. An 11 year-old XSCID patient with no detectable engraftment from prior haploidentical BMTs had lymphocytopenia, growth failure, infections, chronic diarrhea and skin rashes. After G-CSF mobilization and harvest by apheresis, his purified autologous peripheral blood CD34+ cells were transduced daily for 4 days with GALV-MFGS-γc retrovirus in the presence of growth factors and Retronectin®. Eighty million cells/kg (80% CD34+; 40% γc transgene positive) were reinfused. At 1, 2 and 3 months after treatment, provirus marking by PCR of unseparated blood leukocytes was 1.4%, 2.3% and <0.01%, respectively. At 4.5 months, marking reappeared in lineages dependent on IL2RG expression: 0.5% in T cells, 0.1% in NK cells and 0.05% in B cells. This lineage-specific marking persisted at the same level at 6 months. LAM PCR showed polyclonal marking. T-lymphocytes have not yet increased above 300/μl. However, from 2 months after gene therapy the patient experienced a sustained improvement in well-being with resolution of lifelong diarrhea and rashes. No infections have occurred except one episode of otitis externa 3 months post therapy that resolved promptly to oral antibiotics. At six months submandibular lymph nodes were palpable for the first time in his life. Thymus size on chest CT images increased from 1.8 cm3 to 3.2 cm3. Additional follow up is necessary to know if gene marking and clinical improvement persist and if significant expansion of corrected lymphocytes occurs. Our preliminary results suggest that ex vivo retrovirus-mediated gene therapy, targeting CD34+ cells from peripheral blood, may benefit older children with XSCID who have failed haploidentical BMT.