Oncogenes and human neoplasia

Genes whose products are directly involved in the transformation of a normal to a neoplastic cell are present in most but not all oncogenic RNA viruses (retroviruses) have genes found in oncogenic RNA viruses (retroviruses) have also been found in normal cells, their coding sequences highly conserved in evolution. Such viral genes (v-onc) are expressed at high levels in infected cells. "Activation" of cellular homologues of v-onc genes (c-onc) may occur by a variety of mechanisms leading to an abnormal and/or increased expression of such activated c-onc genes in malignant cells. Although oncogene activation appears to be a critical step in the neoplastic transformation induced by oncogenic viruses, the role of this process in the development of chemical and radiation-induced neoplasia is not yet clear.