Carcinoembryonic antigen (CEA) is a glycoprotein and the most commonly used tumor marker in gastrointestinal cancer. Initially, it was thought to be a specific marker for colon cancer, but it has been found that it can also increase in breast cancer, lung cancer, and liver cancer. Although it is not often used as a screening test due to its low positive prediction rate, it can be used to determine the stage, prognosis, and recurrence of colon cancer.