Carcinoembryonic antigen (CEA) is a cell-surface glycoprotein with a molecular weight of 180-200kD, that occurs in high levels in colon epithelial cells during embryonic development. Levels of CEA are significantly lower in colon tissue of adults, but can become elevated when inflammation or tumours arise in any endodermal tissue, including the gastrointestinal tract, respiratory tract, pancreas and breast.An overexpression of CEA protein has been detected in a variety of adenocarcinomas, including gastric, pancreatic, small intestine, colon, rectal, ovarian, breast, cervical and non-small-cell lung cancers.

CEA is also expressed by epithelial cells in several non-malignant disorders, including diverticulitis, pancreatitis, inflammatory bowel disease, cirrhosis, hepatitis, bronchitis and renal failure and also in heavy smokers.