The UL97 protein is a viral orthologue of the cellular cyclin dependent kinases (Cdks) and thus is referred to as a v-Cdk. The UL97 gene is expressed with early/late kinetics and the protein, which localizes to the nucleus, is a minor virion component. UL97 is a kinase that phosphorylates (and thus activates) the antiviral drug gancyclovir, as well as viral and cellular proteins including UL44, Rb, p107, p130, EF-1delta, p32 and lamin A/C. Although not absolutely required for viral replication, null mutants of UL97 show a 10- to 1000-fold growth defect dependent upon the culture conditions of the cells. Defects in the absence of UL97 activity, either by genetic mutation or pharmacological inhibition, include a decrease in viral DNA replication and capsid nuclear egress.