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SCID X1 is characterized by faulty T-cell and natural killer cell differentiation caused by mutation of the γcchain gene encoding a number of multiple cytokine receptors (interleukin-2 [IL-2], IL-4, IL-7, IL-9, and IL-15 receptors). To assess the feasibility of inducing long-term expression and function of the γcchain, Epstein-Barr virus (EBV)-transformed B-cell lines from two patients with SCID X1 were transduced with a Moloney-derived retroviral vector containing the γcchain cDNA. The viral LTR was used as the promoter. Immediately after two cycles of coculture with the Pcrip clone producing the MFG(B2)-γccDNA vector, γcexpression, assessed by detection of the mRNA and membrane protein expression, was found in 15% to 20% of cells. The degree of membrane expression was similar to that in control EBV-B cells. Expression increased steadily over 6 months, becoming detectable in 100% of cells, and remained stable thereafter for a total of 9 months, reflecting positive selection of transduced cells. A study of provirus integration sites showed multiple integration. The expressed γcwas functional, because it restored high-affinity IL-2 receptor binding, IL-2 endocytosis, and IL-2–triggered phosphorylation of JAK-3 tyrosine kinase. Similar results were obtained with the two B-cell lines. These results show that efficient γcgene transfer into B-cells lacking functional γcis feasible and results in strong and stable expression of a functional γcchain, apparently conferring a selective growth advantage in culture. Further in vitro studies of γcgene transfer into γc-hematopoietic progenitors are being conducted to assess the feasibility of correcting lymphocyte differentiation defects.