400 cGy gamma irradiation allows long-term stable engraftment of transduced marrow cells. (A) Five CBA-Pk-1^slc/Pk-1^slc mice underwent transplantation with 10 million 5-FU-treated marrow cells from CBA/N mice transduced with the retroviral vector F36Vmpl^GFP. (B) The schematic shows the experimental plan. (C-F) Two mice received conditioning with 100 cGy (open symbols), and 3 mice received 400 cGy (filled symbols). All 3 mice that were conditioned with 400 cGy before transplantation exhibited stable engraftment of GFP-positive erythrocytes (C) and platelets (D) and correction of their anemia as evidenced by an increase in RBC number (E) and a decrease in the percentage of reticulocytes (F). Mice conditioned with 100 cGy did not have evidence of GFP-positive erythrocytes beyond 10 weeks after transplantation (C; open symbols), did not have GFP-positive platelets (D), remained anemic (E), and had persistently elevated reticulocyte levels (F). Panel G shows the spleens from the 5 mice that received transplants and 2 control mice. The weights are as follows: CBA/N, 60 mg; CBA-Pk-1^slc/Pk-1^slc, 310 mg; 400 cGy, 120 mg, 100 mg, 100 mg; 100 cGy, 350 mg, 360 mg. In panel E, the blue line represents the average RBC number for CBA/N mice, and the red line represents the average RBC number for CBA-Pk-1^slc/Pk-1^slc (Figure 1).