This is an antigen in blood which is elevated in certain cancers of epithelial origin, notably colon, breast, lung and stomach. CEA is a molecule expressed on surfaces of epithelial cells during embryogenesis but is later confined to only the apical surface. Its function is related to cell adhesions. Tumor formation is accompanied with elevated expression of CEA. Blood CEA levels is a tumor marker to detect the recurrence of cancer or residual activity of cancer after surgery. However, CEA is a non-specific marker of cancer, and may be elevated in noncancerous conditions such as cigarette smoking, ulcerative colitis, liver disease, and lung infection. Therefore, CEA is not useful as a screening test. Normal levels of serum carcinoembryonic antigen are 0 to 2.5 ug/L.