Despite recent progress in many areas of treatment and control, cancer remains a frightening threat to everyone. While scientists have known for decades that the majority of human cancers are caused by environmental agents such as radiation and the chemicals in cigarette smoke, not everyone who smokes gets lung cancer. Furthermore, many people who assiduously avoid all possible risk from smoking, diet, and pollution still succumb to some form of cancer later in life. Does this mean that there is an element of blind chance in the underlying mechanisms of human carcinogenesis? To what extent do...