ERBB2 and breast cancer: Furthermore, because the outcomes of HER2‐positive primary breast cancer have substantially improved with the use of dual‐HER2 blockade and trastuzumab emtansine regimens (as shown by the results of the Aphinity and Katherine trials),26, 27 and the number of recurrent cases is decreasing, it is difficult to carry out prospective clinical trials to investigate the role of endocrine therapy in cases of ER‐positive HER2‐positive AMBC.