TOPIC SENTENCES - MAIN IDEA - CONTROLLING IDEA

What causes cancer? The basic causes are unknown, although certain conditions that may lead to cancer have been identified. For example, we know that repeated injury to the cells is a factor. Such damage may result from overexposure to the sun, excessive radiation, and contact with certain chemicals. We also know that some forms of cancer in animals have been caused by viruses. Most physicians and scientists who have studied the problem believe that cigarette smoking is the principal cause of lung cancer. Nevertheless, because of our limited knowledge of what precisely causes cancer, research to find the basic causes of human cancer is still under way in many fields of science.