If a change in HER2 status merely reflects the heterogeneity of HER2 expression within the tumor (i.e., trastuzumab treatment eliminates HER2-overexpressing clones leaving only HER2-negative tumor cells upon completion of therapy), it then follows that trastuzumab should have an equal effect on those tumors achieving a pathological complete response and those tumors becoming HER2-negative. This evidence concerns the gene ERBB2 and neoplasm.