Ganciclovir, a potent inhibitor of the herpesviridae DNA polymerase encoded for on the unique long (UL54) region of the CMV genome, has had a significant impact on both the prophylaxis and treatment of CMV infection in transplant patients. CMV-infected cells produce a phosphotransferase, or kinase (UL97 region) that phosphorylates ganciclovir to ganciclovir-triphosphate. The triphosphate product inhibits CMV replication by competitively inhibiting the incorporation of deoxyguanine triphosphate (dGTP) into the DNA region encoding the polymerase (UL54 region), thus resulting in the premature termination of viral DNA synthesis. Resistance to ganciclovir is conferred when mutations occur on the UL97 region of the CMV genome.