Giant virus—new boundary of virus

Giant viruses are a category of viruses with exceptionally large virions and genomes, both comparable to or even exceeding those of certain prokaryotic cells. Since their first report in 2003, approximately 200 species of giant viruses have been isolated, predominantly infecting unicellular eukaryotic organisms. Taxonomically, these viruses are classified under the phylum Nucleocytoviricota. Despite their complete dependence on host cells for replication and possession of a typical viral replication cycle, giant viruses encode a repertoire of unique genes that have not been identified in other viral taxa, including those associated with core metabolic pathways, protein translation machinery, nucleosome formation, etc. They also encode a large number of repeat domain-containing proteins. This article introduces the main groups and characteristics of giant viruses, their virion structure, genome, and replication mechanisms, with a focus on virus-encoded repeated domains protein genes and special functional genes, as well as the significance of these genes for viral adaptability, the potential ecological value of these viruses, and their relationship with humans, aiming to deepen the understanding of the complexity, evolution, and functions of these viruses.