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.
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