It is now believed that cancer is caused by genetic mutations—most often, by a series of mutations, some of which may be inherited.

Certain normal genes involved in cell growth, development, and differentiation can be converted into cancer-causing "oncogenes" by mutation. Other genes that normally prevent the uncontrolled growth of cells—"suppressor" genes—can also produce cancer if they are knocked out by genetic mutations.

Single mutations are generally not sufficient to cause cancer, but they produce changes that may predispose cells to malignant growth. Additional mutations in other genes, caused by damage from the environment, continue the cells' malignant transformation. Thus, cancer is a multi-step process involving the interaction between genes and their environment.

To test this hypothesis, researchers have put a variety of oncogenes into mice, using promoters to direct the genes to specific tissues. In this way, HHMI investigator Philip Leder and his co-workers at Harvard created and patented "onco-mice"—animals that reliably develop breast and lymph cancers. Onco-mice are being used worldwide to test drugs and therapies against those two forms of cancer.

— Sandra Blakeslee

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