Ras is a GTP-binding protein that is often defective in cancer cells. A common mutation found in cancer cells causes Ras to behave as though it were bound to GTP all the time, which will cause cells to divide inappropriately. From this description, the normal Ras gene is . a proto-oncogene. an oncogene. a proto-tumor suppressor gene. a gain-of-function mutation. a tumor suppressor.