Optimization of retroviral gene transfer into hematopoietic cells of the dog will facilitate gene therapy of canine X-linked severe combined immunodeficiency (XSCID) and in turn advance similar efforts to treat human XSCID. Both canine and human XSCID are caused by defects in the common γ chain, γc, of receptors for interleukin-2 and other cytokines. In this study, normal dogs were given retrovirally transduced bone marrow cells with and without preharvest mobilization by the canine growth factors granulocyte colony-stimulating factor (G-CSF) and stem cell factor (SCF). Harvey sarcoma virus and Moloney murine leukemia virus constructs were used, both containing cDNA encoding human γc. The Harvey-based vector transduced into cytokine- primed marrow yielded persistent detectable provirus in bone marrow and blood and expression of human γc on peripheral lymphocytes. In three dogs, human γc expression disappeared after 19 to 34 weeks but reappeared and was sustained, in one dog beyond 16 months posttransplantation, upon immunosuppression with cyclosporin A and prednisone, with up to 25% of lymphocytes expressing human γc. The long-term expression of human γc in a high proportion of normal canine lymphocytes predicts that retrovirus- mediated gene correction of hematopoietic cells may prove to be of clinical benefit in humans affected with this XSCID.