CEACAM5 and gastric cancer: Biomarkers such as claudin 18.2 (CLDN 18.2), human epidermal growth factor receptor 2 (HER2), mucin 1, natural-killer receptor group 2 (NKG2D), epithelial cell adhesion molecule (EpCAM), mesothelin (MSLN) and carcinoembryonic antigen (CEA) play important roles in the diagnosis and function of gastric cancer (56).