A synthetic guanine derivative with antiviral activity. As the active metabolite of ganciclovir, ganciclovir-5-triphosphate (ganciclovir-TP) appears to inhibit viral DNA synthesis by competitive inhibition of viral DNA polymerases and incorporation into viral DNA, resulting in eventual termination of viral DNA elongation. or An antiviral agent used to prevent or treat cytomegalovirus infections that may occur when the body's immune system is suppressed. In gene therapy, ganciclovir is used with an altered herpes simplex virus-1 gene to kill advanced melanoma cells and brain tumor cells.