CA 19-9, also known as Cancer antigen 19-9 or Sialylated Lewis A, is an antibody antigen first discovered in the sera of patients with gastrointestinal and pancreatic cancers. It is now known that the CA 19-9 antibody binds to sialyl-lewis A, a tetrasaccharide carbohydrate that is highly expressed on the surface of many malignant cancers. There are genetic differences in the ability of certain individuals to express sialyl Lea antigens and such variations should be taken into account when using the CA 19-9 as a tumor biomarker.