This year, an estimated 66,000 women will die of lung cancer in the United States. Cigarette manufacturers were not convinced by these relationships, however compelling they might seem, and continued to argue that the causal connection between lung cancer and smoking had not been proved. Some data pointed to viruses as a potential cause, while other data implicated chemicals of various kinds. Over the next twenty five years, hundreds of laboratories bore down on the problem, and gradually a clear picture emerged. Cancer, it turns out, is a defect in the system cells use to control how frequently they divide.
Cancer results when a gene mutation occurs that stomps on the accelerator or removes the breaks.
The first cancer to be studies in molecular detail, a form of bladder cancer, proved to be a point mutation in a gene specifying a protein that was part of a cell division accelerator. Cancer, then, is the direct result of damage to genes that restrain cell division. Without controls, the mutated cell divides continuously, now a cancer cell. That is why cancer is more common among older people than children. Chemicals that cause mutations in genes are called mutagens.