Cancer results from cell division gone wild. Billions of cells in the adult human body divide every day to replace cells that die. Most of these cell divisions go perfectly well, thanks to an intricate system of checks and balances. But occasionally a cell will defy normal restraints on growth and begin to multiply uncontrollably, giving the body a strong push on the way to cancer.

"A single mutation in a cell's DNA is not enough to cause cancer," points out Bert Vogelstein, an HHMI investigator at The Johns Hopkins Oncology Center in Baltimore, who has tracked the development of colon cancer. "Cancer arises from the accumulation of several mutations," he says. Some genetic mutations are inherited and produce predispositions to specific types of malignancy. Other mutations are caused by environmental factors such as radiation or poisons. These alterations in a cell's DNA may turn normal genes into dangerous oncogenes that accelerate cell growth, or they may inactivate "tumor suppressor" genes that would normally stop unwanted growth. Either way, a cell that has undergone enough mutations may start to proliferate so rapidly that its descendants gain a selective advantage over normal cells, which divide less frequently. Eventually the mutant cells form a tumor.

— Maya Pines

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