Specificity of the DF3 promoter for RPMI 8226 MM cells. DF3-positive RPMI 8226 MM and DF3-negative HeLa cervical carcinoma cells (3 × 10⁴/well) were transduced with Ad.CMV-βgal (A and B; □) and Ad.CMV-tk (C and D, □) or with Ad.DF3-βgal (A and B, ▪) and Ad.DF3-tk(C and D, ▪) at MOI = 0, 1, 5, 10, 50, and 100 for 2 hours. Cells were washed out for 10 hours and tk-transduced tumor cells were then treated with GCV (50 μmol/L) for 36 hours. Comparison of the transduction efficiency (ie, X-Gal staining after transduction with Ad.CMV-βgal or Ad.DF3-βgal) and GCV killing oftk-transduced tumor cells (ie, viable cells by trypan blue exclusion after transduction with Ad.CMV-tk and Ad.DF3-tk followed by GCV treatment) was compared for Ad.CMV versus Ad.DF3 promoters.