The reason for unlimited proliferation of cancer cells is because oncogenes are activated, oncogenes are inactivated, and contact inhibition disappears in cancer cells. In other words, normal cells have contact inhibition after cell-to-cell contact, which means that the cells are restricted from growing without restriction, while cancer cells have no contact inhibition. Moreover, cancer cells can easily fall off from the cancerous tissue and reach another site to grow into a new cancer. Therefore, the growth of cancer cells is not controlled by the body, and it has a characteristic of unlimited proliferation. Since cancer cells have the characteristic of unlimited proliferation, it will cause erosion and consumption to the human body. Therefore, it is clinically necessary to give cancer patients treatment so as to save their lives.