Carbohydrate antigen 19-9 (CA 19-9), also known as sialyl Lewis A, is a glycolipid and O-linked glycoprotein embedded on the surface of cancer cells. It is synthesized by pancreatic cancer cells and is also produced by normal epithelial cells of the pancreas, bile ducts, stomach, and colon. CA 19-9, as the most commonly used and most effective serum tumor marker, can be used for the diagnosis and treatment of pancreatic cancer in symptomatic patients.