TKT and neoplasm: In addition, TK/GCV suicide gene therapy was reported to be enhanced by T cell mediated immune responses and work more efficaciously in syngeneic tumor models compared to xenograft tumor models [52], potentially explaining why oFV-TK/GCV therapy was not superior to oFV-TK alone in prolonging the survival of mice bearing subcutaneous U251-U3-mCherry-U3-luc tumors (data not shown).