Chemotherapy plus the anti-HER2 antibody trastuzumab is the recommended first-line therapy (Muro et al., 2019) based on the randomized, multicenter, phase 3 ToGA (Trastuzumab for Gastric Cancer) trial, in which the overall survival (OS) was significantly longer with chemotherapy plus trastuzumab than with chemotherapy alone (Bang et al., 2010). This evidence concerns the gene ERBB2 and gastric cancer.