HER2 overexpression or gene amplification occurs in approximately 10%–15% of patients with GC.4 In 2010, the phase III ToGA study demonstrated that patients with HER2 overexpressing GC got a survival benefit from treatment with the anti-HER2 recombinant humanized monoclonal antibody, trastuzumab.5 With the success of the ToGA study, trastuzumab was recommended as the first-line treatment in combination with chemotherapy in patients with HER2-positive GC. This evidence concerns the gene ERBB2 and gastric cancer.