6 R: IX= H the presence of the virus-specified thymidine kina~ e.~ These nucleosides are monophosphorylated on the 5'-hydroxyl group4 by this kinase and subsequently converted to the 5'-db5 and then the 5/-triphosphates by cellular kinases. The triphosphate derivative inhibits virus DNA polymerase and thereby inhibits virus replication. 6 Inasmuch as these compounds are poor substrates for cellular kinases, the initial phosphorylation by the virus ...