now applied broadly to any genetic clement associated with cancer induction, including some cellular genes not known to have a viral homolog and relevant genes of the oncogenic DNA viruses which do not have a cellular homolog. In the normal cell, c-twr genes are involved in cellular growth control. The proteins they encode can be assigned to four major classes: growth factors, growth factor receptors, intracellular signal transducers, and nuclear transcription factors (sec Table 11-4),
In 1989 a completely different category of cellular genes called tumor suppressor genes was discovered which also play an essential regulatory role in normal cells. Their protein products are involved in negative regulation of growth. This regulatory role may be ablated by mutation in the tumor suppressor gene. However, because mutations in genes exerting a negative effect are recessive, both copies must be inactivated for excessive activity of a c-o/ic gene to occur, leading to cancer. Although it currently appears that at least half of all cancers may be associated with altered tumor suppressor genes, these genes may be only indirectly involved in most cancers caused by viruses, at a late stage in the multistep process that leads to full-blown malignancy.
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