To address this question, we analyzed the in vitro efficacy of HER2bsFab against two models of trastuzumab refractive breast cancers: MCF-7, an HER2 non-amplified cell line not eligible to trastuzumab treatment due to a weak HER2 overexpression, and JIMT-1, a model of HER2-overexpressing, trastuzumab resistant cell line. Here, ERBB2 is linked to breast cancer.