The Japanese Gastric Cancer Association (JGCA) guidelines, revised in January 2018, recommend capecitabine (or S-1) and cisplatin plus trastuzumab as standard first-line therapy for HER2-positive G/GEJ cancer [7], primarily based on the results of the Trastuzumab for Gastric Cancer (ToGA) study [6]. This evidence concerns the gene ERBB2 and gastric cancer.