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.
Cigarette smoke contains many powerful mutagens.
Cancer biologists propose that the lungs of cigarette smokers are similarly sensitive, and that lung cancer is caused by mutation of growth-regulating genes by mutagenic chemicals within cigarette smoke. The Smoking Gun Cigarette manufacturers claimed to be unimpressed by this work, largely carried out in mice. The tide turned early this year. Scientists studying a tumor suppressor gene called p53 demonstrated a direct link between cigarettes and lung cancer. DNA repair enzymes that fix the trouble.