GCV-induced cytotoxic assay for the determination of the

HSV-Tk transgene expression in the CEM-Tk derived clones. Cells were incubated for 4 days in the presence of increasing concentration of GCV (range, 0.05 to 12.5 μg/mL). Cell proliferation was measured by the incorporation of³H-thymidine into the cellular DNA. Results are expressed as percentage of incorporated ³H-thymidine at each GCV concentration with respect to the incorporation obtained in the absence of GCV. ♦, nontransduced CEM cells; ■, transduced CEM-Tk clones.