ERBB2 and gastric cancer: To select the most efficacious therapy for advanced gastric cancer, including adenocarcinoma of the esophago-gastric junction, it is essential to determine key biomarkers including for example human epidermal growth factor receptor 2 (HER2) expression, programmed death-ligand 1 (PD-L1) combined positive score (CPS), Claudin 18.2, and microsatellite instability (MSI).