Cancer may begin because of the accumulation of mutations involving oncogenes, tumor suppressor genes, and DNA repair genes. For example, colon cancer can begin with a defect in a tumor suppressor gene that allows excessive cell proliferation. The proliferating cells then tend to acquire additional mutations involving DNA repair genes, other tumor suppressor genes, and many other growth-related genes. Over time, the accumulated damage can yield a highly malignant, metastatic tumor. In other words, creating a cancer cell requires that the brakes on cell growth (tumor suppressor genes) be released at the same time that the accelerators for cell growth (oncogenes) are being activated.
![Cancer Tends to Involve Multiple Mutations](https://webarchive.library.unt.edu/eot2008/20090510045902im_/http://www.cancer.gov/images/Documents/4167b7ca-7e27-4eec-9855-640637dde5dc/cancer49.jpg)
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