Deoxypyrimidine kinase is a pyrimidine salvage pathway enzyme in herpesvirus and is a target for antiviral drugs. Its normal substrates are thymidine and dTMP. The enzyme readily phosphorylates the drugs 5-iododeoxyuridine, acycloguanosine (acyclovir), and ganciclovir . When the viral DNA polymerase attempts, in turn, to incorporate them into DNA in place of the corresponding dNTPs all three of the triphosphates of these drugs interfere with DNA replication. Uninfected cells do not efficiently phosphorylate acyclovir and ganciclovir and they phosphorylate 5-iododeoxyuridine only weakly, so DNA replication and virus growth are inhibited selectively in infected cells.