Book excerpt: Colorectal cancer (CRC) is cancer that begins in the colon or rectum of the digestive system. KRAS is a protein that sends signals to cells telling them to divide. In healthy cells, KRAS can be switched on and off. In some cancer cells, gene mutations cause changes in the KRAS protein that mean it is constantly switched on, driving cells to divide. KRAS mutation status can be used to guide treatment decisions. New treatments that target a KRAS mutation have been approved for non-small cell lung cancer and are now being tested for CRC. Other treatments are also being developed.