<p><b>A.</b> Huh7 cells were transfected with pCI-vTK and G418-resistant Huh cellular clones (cl) (1–6) were evaluated for vTK expression (∼43 KDa, arrowheads) by immunoblot analysis. Lmw, low molecular weight markers; 293TvTK, 293T cells transiently transfected with pCI-vTK. <b>B.</b> In vitro cytotoxicity of GCV in vTK expressing cells. HuhvTKcl1 and HuhvTKcl6 cells were incubated with various concentrations of GCV for 72 hours, followed by cell survival quantitation by MTT assay. Data represent the Mean +/− S.D from quadruplicate cell cultures for each GCV dose. Mock, Huh cells that underwent stable selection after transfection with empty pCIneo vector.</p