Type C virogenes: genetic transfer and interspecies transfer.

Somatic cells of vertebrates contain gene sequences which are an integral part of chromosomal DNA and which code for the production of complete type C RNA viruses. These virogenes are genetically transmitted from parent to progeny along with other cellular genes (virogene-oncogene hypothesis). Activation of this endogenous virogene information from a normally repressed state, rather than infection by exogenous oncogenic viruses, has been proposed as the most common mechanism of cancer causation in animals, including man. Recent isolates of baboon type C RNA viruses, while related morphologically and biochemically to other mammalian type C RNA viruses, can be distinguished by nucleic acid hybridization and immunologic criteria. Within primates, type C virogenes have evolved as the species have evolved; virogenes from closely related genera and families have the closest gene sequence homology. Endogenous viruses from one species may infect animals of a distantly related species and become incorporated into their germ line. Genomes of exogenous viruses, such as the murine leukemia viruses, which are infectious from animal to animal within the same species, evolve more rapidly than the endogenous virogenes which replicate solely as cellular genes. A major viral structural protein of baboon type C RNA viruses, p30 was detected by radioimmunoassay in normal primate tissues. Radioimmunoassays have also detected p30 antigen in human tissues which appear to be immunologically related to primate viral p30. Hybridization experiments have confirmed that type C viral sequences are also present in the human genome.