Among these, the combination of trastuzumab with the second HER2 monoclonal antibody pertuzumab has shown substantial effect in HER2-amplifed breast cancer.[9,10] The HER2 pathway also plays a key role in pathogenesis of gastric cancer, in which 1.2% to 9% of patients are HER2-positive.[11] This second indication for trastuzumab has been approved in Japan for HER2-positive advanced or recurrent gastric cancer. This evidence concerns the gene ERBB2 and breast cancer.