Low level of expression b. Expression restricted to certain times during the cell cycle c. Some viruses contain v- onc that are constitutively expressed. In the early 1900s found that retroviruses cause cancer in animals. In 1908 found erythroblastosis in chickens and 1914 found sarcoma on fowl.
Viruses are extremely small microbes that pas through most filters.
There are both DNA and RNA viruses, both of which lack normal cellular organization. Besides the genome, the viruses contain a few proteins that serve as structural components or to help in viral life-cycle. They are all obligate intracellular parasites. Some viruses are cytolytic and rapidly kill the host cell while others are latent and their genome is only replicated when the cell divides. Because viruses have only a few genes, it is simpler to scan the genes of an oncogenic virus for oncogenes rather than try to scan all the genes of humans.