However, after GCV treatment, survival rates of NPC 5-8F and MCF-7 cells transfected with the enhanced plasmid decreased to 0.370 ± 0.024 and 0.462 ± 0.049, respectively, which was significantly lower than that, 0.533 ± 0.020 and 0.515 ± 0.025, of tumor cells NPC 5-8F and MCF-7 transfected with the plasmid pGL3-basic-hTERTp-TK- EGFP and treated with GCV, respectively. Here, TKT is linked to neoplasm.