Patients affected by severe combined immunodeficiency (SCID) have a profound defect in T cell development and the only curative treatment is hematopoietic stem cell transplantation (HSCT). When an HLA-identical family related donor exists, the success rate of HSCT is 90% while in all others cases, it ranges between 40 and 70%. In SCID patients also presenting B cell differentiation defects, such as in RAG-1 and RAG-2 deficiencies, the survival rate is significantly reduced. We have thus been interested in the development of a potential alternative treatment by using retroviral gene transfer of a normal copy of RAG-1 cDNA. To this end, murine RAG-1 deficient hematopoietic stem cells were transduced with a retroviral modified Moloney leukemia virus vector containing the RAG-1cDNA and transplanted into RAG-1-deficient mice. B and T cell development was achieved in all the recipient mice and the reconstitution is stable over time, attesting for a selective advantage of transduced progenitors. The T cell compartment was found to be diverse and functional as shown by Va and Vb TCR immunoscope analysis as well as proliferation assays in the presence of mitogens. The mature circulating B lymphocytes were not abundant but functional as the serum immunoglobulin levels and specific antibody response against the T cell dependent KLH antigen were always within normal range. Noteworthy, a high transgene copy number was detected in all lymphoid organs and this was associated with a risk of insertional mutagenesis as observed in one mouse. Altogether, these results demonstrate that RAG-1 gene transfer can correct immunodeficiency associated with RAG-1 defect but that human application would require the use of lentivirus vectors with self inactivating long terminal repeats in order to decrease the risk of lymphoproliferative diseases.