Cytomegalovirus (CMV) (from cyto- 'cell' via Greek - 'container' + 'big, megalo-' + -virus via Latin 'poison') is a genus of viruses in the order Herpesvirales, in the family Herpesviridae, in the subfamily Betaherpesvirinae. Humans and other primates serve as natural hosts. The 11 species in this genus include human betaherpesvirus 5 (HCMV, human cytomegalovirus, HHV-5), which is the species that infects humans. Diseases associated with HHV-5 include mononucleosis and pneumonia. In the medical literature, most mentions of CMV without further specification refer implicitly to human CMV. Human CMV is the most studied of all cytomegaloviruses. MX2/MXB was identified as a restriction factor for herpesviruses, which acts at a very early stage of the replication cycle and MX2/MXB restriction of herpesvirus requires GTPase activity. Taxonomy Within the Herpesviridae, CMV belongs to the Betaherpesvirinae subfamily, which also includes the genera Muromegalovirus and Roseolovirus (human herpesvirus 6 and human betaherpesvirus 7). It is also related to other herpesviruses within the Alphaherpesvirinae subfamily, which includes herpes simplex viruses 1 and 2 and varicella-zoster virus, and the Gammaherpesvirinae subfamily, which includes Epstein–Barr virus and Kaposi's sarcoma-associated herpesvirus. Several species of Cytomegalovirus have been identified and classified for different mammals. The most studied is Human cytomegalovirus (HCMV), which is also known as Human betaherpesvirus 5 (HHV-5). Other primate CMV species include Chimpanzee cytomegalovirus (CCMV) that infects chimpanzees and orangutans, and Simian cytomegalovirus (SCCMV) and Rhesus cytomegalovirus (RhCMV) that infect macaques; CCMV is known as both Panine beta herpesvirus 2 (PaHV-2) and Pongine betaherpesvirus 4 (PoHV-4). SCCMV is called cercopithecine betaherpesvirus 5 (CeHV-5) and RhCMV, Cercopithecine betaherpesvirus 8 (CeHV-8). A further two viruses found in the night monkey are tentatively placed in the genus Cytomegalovirus, and are called Herpesvirus aotus 1 and Herpesvirus aotus 3. Rodents also have viruses previously called cytomegaloviruses that are now reclassified under the genus Muromegalovirus; this genus contains Mouse cytomegalovirus (MCMV) is also known as Murid betaherpesvirus 1 (MuHV-1) and the closely related Murid betaherpesvirus 2 (MuHV-2) that is found in rats. Species The genus consists of these 11 species: Aotine betaherpesvirus 1 Cebine betaherpesvirus 1 Cercopithecine betaherpesvirus 5 Human betaherpesvirus 5 Macacine betaherpesvirus 3 Macacine betaherpesvirus 8 Mandrilline betaherpesvirus 1 Panine betaherpesvirus 2 Papiine betaherpesvirus 3 Papiine betaherpesvirus 4 Saimiriine betaherpesvirus 4 Structure Viruses in Cytomegalovirus are enveloped, with icosahedral, spherical to pleomorphic, and round geometries, and T=16 symmetry. The diameter is around 150–200 nm. Genomes are linear and nonsegmented,