TKT and cancer: Several studies reported promising therapeutic effects in various tumor models using hypoxia-responsive promoters combined with gene-directed enzyme prodrug therapies (GDEPT) such as bacterial nitroreductase/CB1954 (enzyme/prodrug), HSV-TK/ganciclovir, cytochrome P450 reductase/RSU1069 and bacterial cytosine deaminase/5-fluorocytosine either by applying viral vectors or after transplanting cancer cells expressing a prodrug-activating enzyme (reviewed by Harada [39]).