Knockdown of HPRT allows for selection of marked cells in mixed cultures of transduced and untransduced cells. The puromycin resistance gene was replaced with the gene for GFP in the Ctrl and 491 plasmids, resulting in plasmids CtrlG and 491G. Fl5.12 cells were transduced and sorted for GFP⁺ cells. (A) Western blot analysis of cell lysates from sorted GFP⁺ cells and untransduced cells with antibodies against HPRT, Chk2 (sc-9064; Santa Cruz Biotechnology), GFP (Clontech, Mountain View, CA), and actin. (B) GFP⁺ cells were mixed with untransduced cells at a ratio of 1:10. The mixed cultures were then subject to treatment with 6TG or no drug. The percentage of GFP⁺ cells is plotted against time (P < .001 for 6TG 5 and 10 nM compared with untreated; ANOVA). (C) The same mixed cultures of 491G transduced and untransduced cells were subjected to either constant or intermittent treatment with 6TG or no drug. Intermittent treatment consisted of 48 hours of exposure to 6TG alternating with 96 hours with no drug. The percentage of GFP⁺ cells after 16 days of treatment is shown on the left, whereas the number of population doublings of GFP⁺ cells is graphed as a percentage of the number of population doublings of untreated GFP⁺ cells on the right. n/a indicates constant treatment with 20 nM 6TG resulted in cell death by 10 days, even in cells with knockdown of HPRT. **P < .01; ***P < .001.