To develop gene therapy targeting thyroid carcinoma, the recombinant retrovirus (LNTGTK) carrying herpes simplex virus thymidine kinase (HSV-TK) gene under the control of thyroglobulin (TG) promoter was constructed and its efficacy was investigated in 3 thyroid cell lines; a differentiated normal rat thyroid cell line (FRTL5), malignant rat thyroid carcinoma cells derived from FRTL5 (FRTC) and a human anaplastic thyroid carcinoma cell line (FRO). TG mRNA was detected by Northern blot analysis in FRTL5 cells and by RT-PCR in FRTC cells when cultured with 2 U/L TSH and its expression levels were decreased by TSH withdrawal. However, either methods revealed no TG expression in FRO cells. In vitro cytotoxic assays demonstrated TG expression status-dependent cell killing by transduction of LNTGTK followed by ganciclovir (GCV) treatment. Thus, LNTGTK transduction increased the GCV sensitivity approximately 13,000- and approximately 160-folds in the presence of TSH and approximately 4- and approximately 27-folds in the absence of TSH in FRTL5 and FRTC cells, respectively. In contrast, there was no difference in the GCV cytotoxicity between parental and transduced FRO cells. Significant growth inhibition, but not complete eradication, of transduced FRTC cells was observed in in vivo subcutaneous tumor models of nude mice. These results demonstrate that retrovirus-mediated transduction of HSV-TK gene under the control of the TG promoter confers the GCV sensitivity selectively to TG-expressing thyroid cells. This system may therefore be feasible for gene therapy targeting TG-expressing thyroid carcinomas.