Tumor producing viruses are called oncoviruses. There is convincing evidence that viruses cause a number of specific cancers in animals. RNA tumour viruses are called oncornaviruses because they are oncogenic and contain RNA. They are classified as retroviruses because the RNA viral genome forms a DNA copy, a reversal of the normal DNA-RNA synthesis. Names of four RNA tumor viruses are as follows:
(1) Rous sarcoma virus(RSV) causes chicken sarcoma.
(2) Murine sarcoma viruses(MSV) causes sarcoma in mice.
(3) Murine leukemia viruses(MLV) causes leukemia in mice.
(4) Avian sarcoma viruses(ASV) causes sarcoma in birds.