Acyclovir is an antiviral drug used to treat herpesvirus infection. The basis of its action is that acyclovir resembles part of the guanosine nucleoside and is phosphorylated by the viral enzyme, deoxypyrimidine kinase. The phosphorylated triphosphate form of acyclovir is an inhibitor of the herpesvirus DNA polymerase. A related compound, ganciclovir, works similarly. Uninfected cells do not efficiently phosphorylate acyclovir and ganciclovir, so DNA replication and virus growth are inhibited selectively in infected cells.