Human telomerase reverse transcriptase (hTERT), the catalytic subunit of the telomerase, is transcriptionally upregulated in more than 90% of tumor cells. It may be used as a tool for driving a gene to kill tumors specifically. To test this idea, luciferase reporter gene was used and the results showed that hTERT promoter could restrict the gene expression in the telomerase-positive tumor cells. A tumor-specific expression plasmid phTERT-CD was constructed, in which the E. coli cytosine deaminase (CD) gene was controlled by the hTERT promoter. A colorectal cancer cell line (LoVo) and a normal amnion cell line (WISH) were transfected by this plasmid. It was shown that the expression of the CD gene increased the sensitivity of LoVo cells to the prodrug, 5-fluorocytosine (5FC), over 800-fold, while the sensitivity of WISH cells to 5FC was increased only 6-fold. Mixed cell experiments showed a strong "bystander effect" on CD-negative cells. Furthermore, a significant anti-tumor effect of the phTERT-CD/5FC system was observed in nude mice bearing mammalian carcinoma induced by s.c. inoculation of LoVo cells when the mice were given 250 mg/kg 5FC twice a day for 10 consecutive days. These results indicated that hTERT promoter could target the suicidal effect of CD gene to tumor cells, and therefore, may be a novel and promising targeting approach to the treatment of cancer.