RNA-tumour-virus genes and transforming genes: patterns of transmission.

RNA tumour virus genes are contained in the chromosomal DNA of most vertebrates, and may be transmitted vertically from parent to progeny along with other cellular genes, as well as horizontally as infectious particles. Activation of these viral genes may be part of the means by which RNA tumour viruses produce cancer. Viral genes and their possible gene products have been characterized. The envelope glycoprotein, for example, interacts with specific membrane receptors on cell surfaces and the major phosphoprotein binds to specific viral RNA sequences. Type-C viral gene sequences have evolved as the species have evolved, and have been transferred between distantly related species under natural conditions. The presence of genetically transmitted viral genes in several vertebrate species, including primates, and the evidence that they may provide normal functions beneficial to the species carrying them, suggests that the potential to cause cancer is a pathological manifestation of a normal physiological process.