The most prominent suicide gene used in preliminary studies of oncolytic viral therapy has been the TK gene of HSV-1.3,4,7 Expression of this gene sensitizes virally-transduced cells to selective drugs; in the case of HSV-1 TK, the most commonly used drug is GCV, a nucleoside analog which is 10 times more effective than acyclovir (ACV) in inducing apoptosis.7 The effectiveness of suicide gene therapy rests on the principle of the so-called “bystander effect” in which cancer cells transduced by the virus cause activation of the prodrug. The gene discussed is TKT; the disease is cancer.