MAb104-DX8951 showed significant tumour growth inhibition compared to ADC control (IgG1-DX8951) (p = 0.0022), trastuzumab (p = 0.0043) or mAb104 alone (p = 0.0043) in the HER2-positive (IHC 3+) trastuzumab-resistant breast cancer xenograft model, HCC1954. This evidence concerns the gene ERBB2 and breast cancer.