Despite their known potential for effectively killing cells, the therapeutic use of plant and bacterial toxins for the treatment of cancer has been slow to enter the clinical setting. This has been due in large part to the lack of gene regulatory elements that control expression of highly toxic genes in a sufficiently tight manner, such that the toxins are only expressed in specific target cells. "Leaky" promoters result in unwanted and harmful cell death. In this study, we tested a novel gene therapy strategy aimed at expressing diphtheria toxin (DT-A) in androgen-independent prostate cancer cells that express the protein BCL2. This strategy relies on both transcriptional regulation and inducibly regulated DNA recombination mediated by the site-directed Flp recombinase to control expression of the toxin. Adenoviruses are used to introduce the genetic elements required for this approach into cultured cells and xenografts. Administration of 4-hydroxytamoxifen, resulting in recombination and expression of the toxin, effectively kills the cancer cells. Our results suggest that following androgen ablation therapy for the treatment of prostate cancer, use of a regulated recombination system to target expression of DT-A to androgen-independent cancer cells would be an effective way to arrest the development of recurrent tumors.