RESUMO
The discovery of RNA oncoviruses dates back to 1911 when Rous isolated the avian virus which is the cause of the sarcoma which bears his name and to 1936 when Bittner related the "milk factor" to the development of murine mammary cancer. During the 50s, the successive descriptions of virus-induced sarcoma-leukemias in mice led to the oncogene theory and gradually to the postulation of a viral origin of cancer. The discovery of the reverse transcriptase in 1970 led to the establishment of the Retroviridae family including both onco and lentiviruses. The decade of the 80s was marked by three fundamental discoveries which altered the concept of oncovirus: 1) oncogenes became established as part of the cellular genome converting retroviruses into occasional vectors of the oncogene; 2) as the T cell growth factor, interleukin-2, became available, the first human oncovirus, HTLV-I, was isolated and proved to be the cause of adult T cell leukemia; 3) HIV was isolated and classified as a lentivirus and as the cause of AIDS. A few years later the antioncogenes were discovered. Both oncogenes and anti-oncogenes were found to collaborate in the cell cycle, maintaining an equilibrium between proliferation and apoptosis. Today the viral theory has been replaced by the gene theory of cancer which postulates that neoplastic transformation is the result of a cascade of events which include uncorrected DNA errors, blocking of apoptosis, activation of oncogenes and deletion of antioncogenes. At the present time, the intriguing question for retrovirologists is the role played by endogenous retroviruses which in man occupy up to 0.1 of the cellular genome.