5-Iododeoxyuridine is a derivative of uridine used to combat herpesvirus. The viral enzyme, deoxypyrimidine kinase, readily phosphorylates 5-iododeoxyuridine and the viral DNA polymerase attempts, in turn, to incorporate them into DNA in place of the corresponding dNTP.The drug interferes with DNA replication. Uninfected cells phosphorylate 5-iododeoxyuridine only weakly, so DNA replication and virus growth are inhibited selectively in infected cells.