To determine the tumor cell specific toxic effect of our SB-based gene delivery system, cancer cells and normal fibroblasts were co-transfected with the modified SB system (pT.hTp.HSV-tk.Con) along with the active helper plasmid (pCMV-SB) following the administration of 50 μg/mL gancyclovir (GCV). The gene discussed is TKT; the disease is cancer.