Knockdown of HPRT in primary BM cells provides resistance to 6TG and promotes selection of transduced cells. Primary BM cells were harvested from donor mice and transduced with CtrlG and 491G vectors overnight. The cells were cultured with cytokines in the presence of 6TG (5 nM and 10 nM), cisplatinum (1 μM), or no drug. Cells were counted, analyzed by flow cytometry, and replated every 72 hours. (A,B) The percentage of GFP⁺ cells is plotted against time for CtrlG (A) and 491G (B) transduced cells (P < .001; ANOVA). (C,D) The number of population doublings of GFP⁺ and GFP⁻ cells for each condition was calculated and is represented for CtrlG (C) and 491G (D) transduced cells. **P < .01. ***P < .001.