Carcinoembryonic antigen (CEA) is a type of glycoprotein molecule produced by cells of the gastrointestinal tract during embryonic development. It is generally present in low levels in the blood, and elevated levels of CEA are detectable in certain types of cancer. To date, CEA is the most widely used tumor marker for assessing prognosis, detecting recurrence (when used in combination with other tumor markers), and monitoring treatment in people with colorectal cancer. High levels of CEA are also expressed in cancers of the pancreas, stomach, breast, lung, medullary carcinoma of the thyroid, and ovarian cancer.