TKT and head and neck neoplasm: The HSV-tk/GCV system has been employed to catalyze GCV into its cytotoxic form [i.e., ganciclovir triphosphate (GCV-TP)] by exogenously expressed HSV-tk enzyme for the treatment of brain tumors [9,10], colorectal tumors [11,12], and head/neck tumors [13,14].