To support this hypothesis, we evaluated the cytotoxic activity of HER2bsFab against two different breast cancer cell lines, MCF-7 which weakly overexpresses HER2 and JIMT-1 which, although overexpressing HER2, displayed a low trastuzumab-binding capacity due to overexpression of mucin 4 (MUC4) [32]. This evidence concerns the gene ERBB2 and breast cancer.