BMC Cancer 2006, 6:66 http://www.biomedcentral.com/1471-2407/6/66 alter expression in response to disease progression, growth and development of younger patients, or unto- ward side-effects. Regulating gene expression by exoge- nously administered drugs provides control over the level of functional gene product. Regulation permits much greater control over timing of gene expression and can provide a baseline level of behavior or cellular function in the presence of Dox. To have control over the time and level of gene expression is essential for both the in vitro and in vivo studies of gene function, especially for the safety and efficacy of gene therapy. The HSVtK gene expression in tumor cells of nude mice was detected by RT-PCR and there was a faint HSVtK gene expression without Dox induction, as shown in our data. It may be due to leakage of the Tet-On regulatory system [34], but this does not influence the efficiency of the treat- ment as it has only lower suicide gene expression com- pared with the experimental group. Sometimes the therapeutic gene is still expressed when Dox does not exist because there is a residual affinity between TRE and rtTA units of the system. So many measures are taken using a certain modified rtTA system which has shown promising results in low basal expression in the absence of inducer, high induced expression, high induction ratio, indiffer- ence to endogenous cellular processes, low immunogenic- ity of system components and safety of inducers [35-38]. We constructed rAAV vectors containing Tet-On system and HSVtk gene to explore the effect of breast cancer treat- ment. Our results show that the expression of HSVtk gene was controlled by Tet-On system under the induction of Dox, transduced by rAAV. The antitumor treatment occurred with an acceptable toxicity level, and the treated mice achieved long-term survival. The effectiveness of the HSVtk/Tet-On/GCV system would provide an effective reg- ulatory treatment strategy for breast cancer including other solid tumors. Conclusion In conclusion, our data demonstrated that tumor growth of BALB/C nude mice breast cancer was inhibited with direct intratumoral injections of rAAV-2/HSVtk/Tet-On under the Dox induction. The expression of HSVtk gene in MCF-7 cells and BALB/C nude mice tumors was up-regu- lated by Tet-On under Dox induction. Our studies suggest that rAAV-mediated HSVtk/GCV gene therapy under the Dox induction with direct intratumoral injections may be a useful method in treatment for breast cancer and other solid tumors. Abbreviations AAV (adeno-associated virus); HSVtk (herpes simplex virus thymidine kinase); GCV (ganciclovir); Dox (doxycy- cline); v.p (viral particle); rtTA (reverse tetracycline trans- activator); TRE (tetracycline-responsive element); tet (tetracycline); DMEM (Dulbecco's modified Eagle's medium); FBS (fetal bovine serum). Competing interests The author(s) declare that they have no competing inter- ests. Authors' contributions LI Zi-Bo is responsible for the generation of rAAV particles expressing HSVtK and Tet-On, rAAV infections and MTT assay. ZENG Zhao-Jun carried out animal experiments and contributed to the evaluation of treatment effects and is co-first author. CHEN Qian and LUO Sai-Qun contrib- uted to the construction of rAAV vector. HU Wei-Xin, as the corresponding author, designed the protocol and made the draft of the manuscript. All authors read and approved the final manuscript. 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