CEACAM5 and digestive system neoplasm: Glycoproteins, such as carcinoembryonic antigen (CEA), cancer antigen 19-9 (CA19-9), carbohydrate antigen 15–3 (CA 15–3), and carbohydrate antigen 12–5 (CA 12–5), are classical circulating biomarkers for GI tumors [7].