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Many chromosomal and point mutations have been described in cancer cells. Such mutations affect *oncogenes, whose products stimulate cell division, or tumor suppressor genes, whose products inhibit cell division. More than two gene mutations are usually needed for full-blown cancer.

Because colon cancer progresses to full malignancy slowly, it has been possible to identify the gene mutations that lead to each stage. Figure 15.10 outlines the “molecular biography” of this form of cancer. At least three tumor suppressor genes and one oncogene must be mutated in sequence in a cell in the colon lining for cancer to develop. Although the occurrence of all of these events in a single cell might seem unlikely, remember that the colon lining has millions of cells, that these cells arise from stem cells that are constantly dividing, and that these changes take place over many years of exposure to natural and synthetic substances in foods, which may act as mutagens.