CEACAM5 and gastric cancer: Serum carcinoembryonic antigen (CEA) and carbohydrate antigen (CA) 19-9 are the most common tumor markers targeting multiple cancers, including colorectal cancer, gastric cancer, pancreaticobiliary cancer, lung cancer, and breast cancer; they are used in cancer care as prognostic markers and markers for the monitoring of response to therapy and recurrence2–6.