When runaway cell division occurs, it does not necessarily lead to cancer. Neighboring cells respond by excreting a growth inhibitor. This chemical binds to receptors in the malfunctioning cell, sending a signal to the nucleus that activates tumor suppressor genes. Tumor suppressor genes are like brakes for cell growth. When activated, these genes halt the cell cycle, preventing further cell division.
But if tumor suppressor genes malfunction due to mutations, the rapidly dividing cell ignores messages from its neighbors telling it to stop dividing. Malfunctioning tumor suppressor genes are not enough to cause cancer—the cell still must overcome a host of other safety mechanisms before it can cause truly significant damage.
Tumor suppressor genes are genes that have the power to stop (or suppress) tumors from forming. They do that by preventing cells from dividing uncontrollably. There are many of these kinds of genes in the human DNA. Normally people inherit two working copies of these cancer-protector genes. They get one from each parent. Sometimes, though, a person inherits only one working copy. If that gene stops working, the person is more at risk for cancer.