ERBB2 and breast cancer: Trastuzumab in combination with pertuzumab and docetaxel has been approved for the treatment of patients with HER2-positive metastatic breast cancer, but trastuzumab in combination with pertuzumab and chemotherapy in the first-line treatment of HER2-positive metastatic gastric or gastro-esophageal junction cancer showed negative results in a phase III trial [5, 33], suggesting that there are intrinsic differences in the tumor biology of HER2-positive advanced gastric cancer and HER2-positive breast cancer in response to HER2 dual blockade.