Conventional plasma/serum-based tumor biomarkers commonly used clinically for early GC diagnosis, including carcinoembryonic antigen (CEA), the carbohydrate antigens (CA), CA19-9, CA72-4, CA125, CA24-2 and CA50, as well as pepsinogen and α-fetoprotein (AFP), have poor specificity and sensitivity [8,9]. Here, CEACAM5 is linked to gastric cancer.