Although there are several commonly used serum biomarkers such as alpha-fetoprotein (AFP), carcinoembryonic antigen (CEA), cancer antigen 125 (CA125), and cancer antigen 19–9 (CA19-9) (He et al., 2013) for gastric cancer diagnosis, none of them are sensitive or gastric-cancer-specific (Smyth et al., 2016). Here, CEACAM5 is linked to gastric cancer.