However, inter-lesional HER2 heterogeneity may be associated with poor response to HER2-targeted therapy including trastzumab, as intra-tumoural HER2 heterogeneity.27 Although the clinical significance of subtype discordance in multiple breast cancer has not been determined yet, recognition of inter-lesional heterogeneity of standard biomarkers such as HER2 may provide additional information on treatment response in those patients. The gene discussed is ERBB2; the disease is breast cancer.