Together with the findings in this study that alternative driver genetic alterations may be present in the HER2-negative components of HER2 heterogeneous breast cancers, our data may provide an explanation for the presence of discordant HER2 status between primary tumors and metastases when assessed on biopsy material, which has been found in up to 14% of cases [53]. This evidence concerns the gene ERBB2 and breast carcinoma.