A study of early stage HER2+ breast cancer patients treated with neoadjuvant T-DM1 and pertuzumab found that the pre-treatment presence of HER-2 heterogeneity, defined as ERBB-2 amplification in 5–50% of tumor cells or an area that was HER2-FISH negative, was inversely predictive of treatment response. This evidence concerns the gene ERBB2 and breast cancer.