This enzyme/prodrug combination could promote cell death, not only in recipient cells, but also in neighboring cells, to kill tumor cells through the so-called “bystander effect” [78, 79] constructed CRAd (Ad-ETK) expressing E1A and HSV-TK genes and confirmed that Ad-ETK enhanced OV therapy for HCC [80]. The gene discussed is TKT; the disease is hepatocellular carcinoma.