Abstract
Recent studies, primarily with mouse, rat, and chicken cells, have provided evidence to support the concept that vertebrates contain the genetic information for producing a type-C RNA tumor virus in an unexpressed form in their somatic cells as well as in their germ cells. This information, which our associates and we postulated has been part of the genetic makeup of vertebrates since early in evolution, can persist for hundreds of generations in cell culture without overt production of virus. It is proposed that the endogenous virogenes (the genes for the production of type-C viruses) and the oncogenes (that portion of the virogene responsible for transforming a normal cell into a tumor cell) are maintained in an unexpressed form by repressors in normal cells. Various agents, including radiation, chemical carcinogens, and, perhaps, exogenously added viruses, may transform cells by "switching on" the endogenous oncogenic information. Some other implications of the viral oncogene theory are presented.