Only a single gene encoding the putative ATPase subunit of the terminase ("UL15") is conserved among all herpesviruses. To a lesser extent this gene is also found in T4-like bacteriophages suggesting a common ancestor for these two groups of viruses. Another paper has also suggested that herpesviruses originated among the bacteriophages. A common origin for the herpesviruses and the caudoviruses has been suggested on the basis of parallels in their capsid assembly pathways and similarities between their portal complexes, through which DNA enters the capsid. These two groups of viruses share a distinctive 12-fold arrangement of subunits in the portal complex. A second paper has suggested an evolutionary relationship between these two groups of viruses. It seems likely that the tailed viruses infecting the archaea are also related to the tailed viruses infecting bacteria. A study involving 600 herpes genomes and 2000 caudoviral genomes suggested that an evolutionary relationship exists between these order. The NCLDV group ("Asfarviridae", "Iridoviridae", "Marseilleviridae", "Mimiviridae", "Phycodnaviridae" and "Poxviridae") along with three other families ("Adenoviridae", "Cortiviridae" and "Tectiviridae") and the phage Sulfolobus turreted icosahedral virus and the satellite virus Sputnik all possess double β-barrel major capsid proteins suggesting a common origin. Several studies have suggested that the family "Ascoviridae" evolved from the "Iridoviridae". A study of the Iridoviruses suggests that the Iridoviridae, Ascoviridae and Marseilleviridae are related with Ascoviruses most closely related to Iridoviruses. The family "Polydnaviridae" may have evolved from the "Ascoviridae". Molecular evidence suggests that the "Phycodnaviridae" may have evolved from the family "Iridoviridae". These four families ("Ascoviridae", "Iridoviridae", "Phycodnaviridae" and "Polydnaviridae") may form a clade but more work is needed to confirm this. Some of the relations among the large viruses have been established. Mimiviruses are distantly related to Phycodnaviridae. Pandoraviruses share a common ancestor with Coccolithoviruses within the family Phycodnaviridae.