ERBB2 and metastatic malignant neoplasm: HER2-positivity by IHC/ISH ASCO/CAP guidelines is based on their predictive value for trastuzumab efficacy in breast and GEA cancers, but likely does not capture the breadth of metastatic cancer patients who may benefit from the remarkable efficacy our expanding arsenal of HER2 agents that possess different activity profiles to trastuzumab.