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cancer: Tumor Development

Most bodily insults by carcinogens come to nothing because DNA has built-in repair mechanisms, but repeated insults can eventually result in mutations or altered gene expression in key genes called oncogenes and tumor-suppressor genes. Oncogenes produce growth factors, substances that signal a cell to grow and divide into daughter cells; tumor-suppressor genes (such as the p16, p53, and BRCA1 genes) normally produce a negative growth factor that tells a cell when to stop dividing. The abnormally inactivated tumor-suppressor gene or the abnormally activated oncogene is inherited by each of the cell's daughter cells, and a tumor develops. In many cases tumors remain small and in one place (in situ) for years, but some develop their own blood vessels (a process known as angiogenesis) and begin to grow and spread.

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

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