In virology, realm is the highest taxonomic rank established for viruses by the International Committee on Taxonomy of Viruses (ICTV), which oversees virus taxonomy. Four virus realms are recognized and united by specific highly conserved traits: Duplodnaviria, which contains all double-stranded DNA (dsDNA) viruses that encode the HK97-fold major capsid protein; Monodnaviria, which contains all single-stranded DNA (ssDNA) viruses that encode a HUH superfamily endonuclease and their descendents; Riboviria, which contains all RNA viruses that encode RNA-dependent RNA polymerase and all viruses that encode reverse transcriptase; and Varidnaviria, which contains all dsDNA viruses that encode a vertical jelly roll major capsid protein.