Proto-oncogenes can be classified into many different groups based upon their normal function within cells or based upon sequence homology to other known proteins. As predicted, proto-oncogenes have been identified at all levels of the various signal transduction cascades that control cell growth, proliferation and differentiation. The list of proto-oncogenes identified to date is too lengthy to include here, therefore, only those genes that have been highly characterized are described. Recessive and the genes variously termed tumor suppressors, growth suppressors, recessive oncogenes or anti-oncogenes. This suspicion has proven true with the identification of numerous genes, whose products function in cellular signaling, that are involved in some way in the genesis of the tumorigenic state.
Tumor viruses are of two distinct types.
RNA tumor viruses are common in chickens, mice and cats but rare in humans. Retroviruses can induce the transformed state within the cells they infect by two mechanisms. Both of these mechanisms are related to the life cycle of these viruses. The DNA then integrates into the genome of the host cell where it can remain being copied as the host genome is duplicated during the process of cellular division. The LTRs promote the transcription of the viral DNA leading to the production of new virus particles.