Zeta’s mouse monoclonal antibody recognizes CA19-9, a carbohydrate epitope expressed on a high MW (>400kDa) mucin glycoprotein. CA19-9 is a sialyl Lewis a structure which is synthesized from type 1 blood group precursor chains and is present in individuals expressing the Lewis a and/or Lewis b blood group antigens. In normal tissues, CA19-9 or sialyl Lewis a antigen is present in ductal epithelium of the breast, kidney, salivary gland, and sweat glands. CA19-9’s expression is greatly enhanced in serum as well as in the majority of tumor cells in gastrointestinal (GI) carcinomas, including adenocarcinomas of the stomach, intestine, and pancreas. Preoperative elevated CA19-9 levels in patients with stage I pancreatic carcinoma decrease to normal CA19-9 values following surgery. When used serially, CA19-9 can predict recurrence of disease prior to radiographic or clinical findings.