Moreover, it has also been established that two forms of “bystander tumour cell killing” mechanisms are mediated by this method: (a) a local “direct” bystander effect, as a result of the transfer of ganciclovir triphosphate from HSV-TK-positive tumour cells into untransfected neighbouring tumour cells [9–11], (b) a non-local systemic immunologically-mediated bystander effect due to the in vivo immune stimulation/presentation of tumour-specific or associated antigens following the killing of HSV-TK-expressing tumour cells [12, 13]. This evidence concerns the gene TKT and neoplasm.