ABSTRACT
Evidence derived from a multitude of studies clearly indicates the existence of important structural and functional relationships between growth factors and oncoproteins. 1-5 The oncoproteins are the products of oncogenes and proto-oncogenes, which are defined as genes with potential properties for the induction of neoplastic transformation in either natural or experimental conditions. 6 Classic oncogenes were isolated from acute retroviruses, which are defective, nonreplicating viruses that act as oncogene transducers (Table 9.1). Acute retroviruses are not infectious under natural conditions but have been isolated from different types of animal tumors and are originated from recombination events that may occur between cellular sequences, called proto-oncogenes, and sequences derived from various infectious chronic retroviruses (Figure 9.1). No transforming sequences (oncogenes) are contained in chronic retroviruses. The viral oncogenes (v-onc genes) contained in acute retroviruses are responsible for the high oncogenic potential exhibited by these viruses under experimental conditions. The v-onc genes are derived from sequences of cellular origin, the proto-oncogenes or c-onc genes. Some putative proto-oncogenes, represented by cellular sequences with oncogenic potential, have not been detected in acute retroviruses but have been isolated directly from cellular genomes.
