Suicide genes (SG) can be viewed as negatively selectable marker genes. Their gene products convert otherwise nontoxic prodrugs (PD) into cytotoxic metabolites. The enzymatic conversion leads to high levels of the activated toxic drug within genetically modified cells and ultimately kills the cells. Several studies have shown that when SG-expressing tumor cells are mixed with parental, nongenetically modified cells, prodrug treatment induces toxic effects in the nonmodified cells. This phenomenon has been termed “bystander (killing) effect.”