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Herpesviruses, such as human cytomegalovirus, are large double stranded DNA viruses that infect most species throughout the animal kingdom. They encode a protease that is essential for production of infectious virons: this enzyme catalyses the maturational processing of the herpesvirus assembly protein.

Human cytomegalovirus (HCMV) protease is a serine protease (member of the peptidase S21 family) that is involved in the proteolytic processing of the assembly protein precursor during capsid maturation. CMV protease shares no sequence homology with classical serine proteases and contains a Ser-His-His catalytic triad, rather than the classical Ser-His-Asp/Glu. The mechanism, however, is much the same. HCMV protease is involved in autoproteolytic cleavage at two sites to produce an enzyme with full catalytic activity. The enzyme cleaves between Ala-Ala/Ser. HCMV protease exists in a monomer-dimer equilibrium, with the homodimer being active and the monomer not. HCMV protease is a potential antiviral drug target.