ERBB2 and neoplasm: Indeed, HER2 heterogeneity, defined as an area with ERBB2 amplification in > 5% but < 50% of tumor cells, or a HER2-negative area by FISH, showed complete response rates of 0% versus 55% in the non-heterogeneous subgroup with neoadjuvant therapy combining T-DM1 and pertuzumab [85].