Most cancers have mutations in proto-oncogenes, the normal genes involved in the regulation of controlled cell growth. These genes encode proteins that function as growth factors, growth factor receptors, signal-relaying molecules, and nuclear transcription factors (proteins that bind to genes to start transcription). When the proto-oncogene is mutated or overregulated, it is called an oncogene and results in unregulated cell growth and transformation. At the cellular level, only one mutation in a single allele is enough to trigger an oncogenic role in cancer development. The chance that such a mutation will occur increases as a person ages.
![Abnormal Cell Growth: Oncogenes](https://webarchive.library.unt.edu/eot2008/20081006123416im_/http://www.cancer.gov/images/Documents/a3a0ee76-1bc5-4490-aeaa-2248702c668f/cancer51.jpg)
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