ERBB2 and breast cancer: Since benefit of HER2 targeted therapies in non-amplified tumors with intermediate levels of HER2 expression (HER2-low), not eligible for HER2-targeted therapies, is still a debated issue26–28, we took advantage of our luminal B breast cancer PDX-BRS18 expressing ER and HER2 (score 2 +, without gene amplification), as a model for the evaluation of the impact of HER2-targeted combined therapies.