Defects in the gene for the IL-7 receptor (R) a chain are one cause of severe combined immunodeficiency disease (SCID) based on a strict requirement for IL-7 in T lymphoid development and survival. We tested the feasibility and potentially undesirable consequences of IL-7Ra gene transfer as a therapy for this genetic defect. The murine IL-7Ra gene was introduced into IL-7Ra À/À bone marrow progenitors using retrovirus and transplanted into Rag À/À recipient mice. Both ab and gd T cells were reconstituted in thymus and spleen showing proof of principle. B-cell development was also restored in some mice, but their numbers were much lower than in the T-cell compartment. Splenomegaly was observed due to an increase in neutrophils. We showed that hematopoeitic progenitors, after transfection with IL-7Ra, could respond to IL-7 in vitro by a striking production of neutrophils and other myeloid cells. These data indicate that although IL-7 is a critical lymphopoietin, ectopic expression of its receptor on multipotential progenitors can also induce production of myeloid cells, presumably through survival and proliferation signals that are not restricted to lymphoid cells. This supports the stochastic model of progenitor differentiation, in which cytokines give permissive and not instructive signals.