Abstract
It was acknowledged long ago that viruses may cause cancer in animals. In 1911, Peyton Rous described viruses as causing sarcomas in poultry. The tumour-inducing virus responsible was later named after him, Rous sarcoma virus. In the following decades, a large number of viruses were discovered that can cause various cancers in poultry and rodents, such as lymphomas, sarcomas and carcinomas. Many of them belong to the family Retroviridae, and were classified into the genera Alpharetrovirus, Betaretrovirus and Gammaretrovirus. Most of these pathogens were isolated from inbred strains of the respective species or from cell cultures; under natural conditions, these strains are likely irrelevant as a cause of cancer in the corresponding species. An exception is feline leukaemia virus (10.1007/978-3-642-20718-1_18). The tumorigenic potential of oncogenic retroviruses is based on transformationally active proteins. They are similar to cellular products which are ordinarily involved in the regulation of cell division. In contrast to the cellular products, viral oncogene proteins are altered by mutations in such a way that they are not subject to regulatory control, and are thus constitutively active. In fact, the discovery of viral oncogenes was pioneering and has paved the way for deciphering cellular oncogenes, and thus for understanding the molecular basis of carcinogenesis. Evidence for the existence of retroviruses that cause cancer in humans was found only in 1982 when Robert Gallo discovered human T-lymphotropic virus (HTLV; 10.1007/978-3-642-20718-1_18).