CEA, or carcinoembryonic antigen, is a family of highly related glycoproteins, of which 18 are normally expressed in humans. Structurally, CEA is a glycosylphosphatidylinositol (GPI) protein anchor and is adhered to the cell membrane. It is produced in gastrointestinal tissue in prenatal fetal development and drops to low levels in healthy adults. In certain cancers, elevated serum CEA has been observed and it has thus been used as a tumor biomarker. Other findings, however, have questioned its reliability in clinical screening or diagnosing colon cancer.