Gene therapy is defined as a technology aimed at modifying the genetic component of cells for therapeutic benefit. 'Suicide genes' can be introduced into cancer cells to make them more sensitive to chemotherapeutics or toxins. Chemotherapeutic suicide gene therapy approaches are known as gene-directed enzyme prodrug therapy or gene-prodrug activation therapy. Other approaches include replacement gene therapy, antisense strategies and induction of resistance to normal cells. All gene therapy strategies share a common component, which is the need for a selective delivery vehicle or vector with tumor-targeting capabilities. This need has led to the in-depth investigation of viruses as new vectors for gene therapy.