Abstract X-linked SCID patients are deficient in functional IL-2Rγc leading to the loss of IL-2/IL-4/IL-7/IL-9/IL-15/IL-21 signaling and a lack of NK and mature T cells. Patients treated with IL-2Rγc gene therapy have T cells develop; however, their NK cell numbers remain low, suggesting antiviral responses may be compromised. Similarly, IL-2Rγc−/− mice reconstituted with IL-2Rγc developed few NK cells, and reconstituted T cells exhibited defective proliferative responses suggesting incomplete recovery of IL-2Rγc signaling. Given the shift toward self-inactivating long terminal repeats with weaker promoters to control the risk of leukemia, we assessed NK and T cell numbers and function in IL-2Rγc−/− mice reconstituted with limiting amounts of IL-2Rγc. Reconstitution resulted in lower IL-2/-15–mediated STAT5 phosphorylation and proliferation in NK and T cells. However, TCR costimulation restored cytokine-driven T cell proliferation to wild-type levels. Vector modifications that improved IL-2Rγc levels increased cytokine-induced STAT5 phosphorylation in both populations and increased NK cell proliferation demonstrating that IL-2Rγc levels are limiting. In addition, although the half-lives of both NK and T cells expressing intermediate levels of IL-2Rγc are reduced compared with wild-type cells, the reduction in NK cell half-live is much more severe than in T cells. Collectively, these data indicate different IL-2Rγc signaling thresholds for lymphocyte development and proliferation making functional monitoring imperative during gene therapy. Further, our findings suggest that IL-2Rγc reconstituted T cells may persist more efficiently than NK cells due to compensation for suboptimal IL-2Rγc signaling by the TCR.