Abstract: A herpes virus proteinase has been found to be encoded by a member of a family of four nested genes in simian cytomegalovirus. Another member of the nested genes encodes the assembly protein precursor, which is a substrate for the proteinase. Homologous genes are found in other herpes viruses. Cleavage sites recognized by the proteinase are identified in cytomegalovirus and are found to be highly conserved in other herpes viruses. Substrates, inhibitors, assay kits, and methods of assaying are provided which rely on the proteinase and its activity.