Cytomegalovirus is a member of the herpes virus group, which includes herpes simplex virus types 1 and 2, varicella zoster virus (which causes chicken pox), and Epstein Barr virus (which causes infectious mononucleosis). These viruses share a characteristic ability to remain dormant within the body over a long period. CMV viral genes are co-ordinately expressed in groups at various times after infection. Early viral proteins are expressed in the nucleus of infected cells within 3 to 24 hours of infection prior to the commencement of viral DNA replication. This is followed by expression of the early intermediate genes, which encode enzymes required for viral DNA replication. After 48 to 72 hours, a number of late viral antigens may be demonstrated in the nuclei and cytoplasm of infected cells. pp65 is a 65kD phosphorylated glycoprotein and is the most abundant of the late antigens.