Adding pertuzumab to trastuzumab and chemotherapy has been shown to significantly improve clinical outcomes in patients with HER2-positive early and metastatic breast cancer (BC) and the combination has been approved for patients with HER2-positive colorectal cancer in Japan based on the results of the TRIUMPH study [2–6]; however, there are several differences in tumor biology between BC and GC, including a higher incidence of HER2 heterogeneity and incomplete membrane staining in GC [7]. The gene discussed is ERBB2; the disease is gastric cancer.