Sialylated Lewis Antigen (CA19-9,Carbohydrate antigen, FUT) is a tetrasaccharide that constitutes the sialytated form of Lewis Antigen A. CA19-9 attaches to O-glycans on the plasma membrane where it functions in intercellular recognition. It is a common pancreatic and metastatic colorectal cancer marker because it shows elevated levels alongside CEACAM5 (CEA) expression in late tumors (Vukobrat-Bijedic, 2013; Polat, 2014). CA19-9 is an effective marker for circulating tumor cells, as increased levels of CA19-9 in serum coincide with the quantity of circulating tumor cells from colorectal metastases (Zhao, 2017). Pre- and post treatment levels of CA19-9 are also used as a guage for the efficacy of a given therapy and as a red flag for reoccurrence. In immunohistochemistry, staining of antibodies to CA19-9 is primarily cytoplasmic.