SEE http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/O/Oncogenes.html

RNA Tumor Viruses (all are Retroviruses)

There are two types of RNA tumor viruses:

1. acutely transforming retroviruses - have viral oncogenes as part of their genome.
2. chronically transforming retroviruses do not have v-oncogenes, but can affect the transcription of cellular protooncogenes by either of two mechanisms
   1. oncogensesis by promotor insertion
   2. oncogenesis by enhancer insertion

Oncogenes in retroviruses (acutely transforming)

About forty oncogenes have now been identified. They are referred so by a three letter code (e.g. src, myc) reflecting the virus from which they were first isolated. Some viruses can have more than one oncogene (e.g. erbA, erbB). Here are a few of the most studied:

CELLS  HAVE PROTO-ONCOGENES
After retroviral oncogenes had been discovered, hybridization of retroviral oncogenes probes to animal DNAs, including humans, demonstrated that copies of many of these genes are present in the genomes of these animals. Unlike the situation with DNA virus oncogenes, which are true viral genes (E6, E7 of papilloma virus), there are homologs of all retrovirus oncogenes in cells that are not infected by a retrovirus. These cellular homologs are genes involved in growth control and development/differentiation and have important non-transforming functions in the cell.

The normal cellular homologs of viral oncogenes are called proto-oncogenes.In their normal genetic state, protooncopenes are important components of cellular signalling and transcription activation. If a protooncogene under goes a somatic mutation, control of cell growth is lost in the cell in which the mutation occurs and cancer can occur. Protooncogenes that have mutated in an individual are called cellular oncogenes and are designated by the prefix "c" (i.e. c-myc), to distinguish them from the viral oncogenes.Those protooncogenes that have not mutated are designated "n" (i.e. n-myc). To distinguish viral oncogenes from cellular proto-oncogenes, they are often referred to as v-onc and c-onc respectively. Note: c-oncs are not identical to their corresponding v-oncs.The virus have picked up a cellular growth controlling or differentiation  genes and, after these genes wer acquired, long ago, they were mutated to their present form.

Summary: cellular protooncogenes can mutate to cellular oncognes, or in the past were picked up by viruses and mutated to viral oncogenes.