高普考/三四等/高員級◆英文題庫下載題庫

Sometimes the normal processes of cell growth, death, and regulation get completely deranged. Cells are transformed and begin robbing their neighbors’ blood supply, like vampires. They defy normal limitations on growth, multiply unceasingly, and then invade space reserved for others. They refuse to die on schedule. These are cancer cells, and they are responsible for about two million new cases of various cancers that occur every year in the United States, and over half a million deaths. One out of five people in industrialized countries will die of cancer, most of them from tumors that originated in a lung, breast, prostate gland, colon, or rectum. And except for a few very clear instances such as lung cancer caused by smoking or exposure to certain chemicals and radiation, no one knows exactly why. Unlike smallpox or malaria, cancer is not a distinctive disease with a single cause. It arises from various kinds of mutations in a cell’s DNA. Some mutations occur naturally. In the course of an average human lifetime, billions of trillions of cell divisions take place. Given this huge number of replications, mistakes are bound to happen. They may result in a disadvantageous change for the organism; some may even spawn cancers. More often, mutations and cancers are prompted by external causes.