Cancer antigen 19-9 (CA19-9), also known as the carbohydrate antigen Sialyl-Lewis(a) (sLe(a)), is widely expressed on epithelial tumors of the gastrointestinal tract and breast and small-cell lung cancer. Overexpression of sLe(a) appears to be a key event in invasion and metastasis of many tumors, and CA19-9 is elevated in most patients with advanced pancreatic cancer. Still, it may also be elevated in other cancers and diseases such as bowel cancer, lung cancer, gall bladder cancer, gall stones, pancreatitis, cystic fibrosis, and liver disease. CA19-9 is used to diagnose and monitor pancreatic cancer progression, and high levels are associated with a poor prognosis. CA19-9 assays are based on the two-site sandwich ELISA or CLIA assay principle.