X-linked severe combined immunodeficiency (XSCID) is caused by mutations in the IL-2 receptor gamma chain (IL2RG) gene, resulting in absent T lymphocytes and nonfunctional B lymphocytes. Recently T lymphocyte production and B lymphocyte function were restored in XSCID patients infused with autologous stem cells transduced with a retrovirus containing the human IL2RG cDNA. To optimize the expression of human IL2RG for future clinical trials, we compared five retroviral vectors expressing human IL2RG from different LTR enhancer-promoter elements in a mouse model. Northern and Southern blot analysis of hematopoietic tissues from repopulated mice revealed that the retroviral vector with the highest expression per copy number was MFG-S-hIL2RG, followed by MND-hIL2RG. All five vectors were capable of restoring lymphopoiesis in irradiated XSCID mice transplanted with transduced IL2RG-deficient hematopoietic stem cells. Transduction of IL2RG-deficient hematopoietic stem cells with all five vectors restored T lymphopoiesis in transplanted stem cell-deficient W/W(v) mouse recipients. However, only XSCID stem cells transduced with the MFG-S-hIL2RG vector generated B lymphocytes in W/W(v) mice. We conclude that the MFG-S-hIL2RG vector provides the best opportunity for in vivo selection and development of B and T lymphocytes for human XSCID gene therapy.