Human Breast Cancer Cells Selected for Resistance to Trastuzumab <i>In vivo</i> Overexpress Epidermal Growth Factor Receptor and ErbB Ligands and Remain Dependent on the ErbB Receptor Network

These results are consistent with the inability of trastuzumab to block the heterodimerization of HER2 and suggest that amplification of ligand-induced activation of ErbB receptors is a plausible mechanism of acquired resistance to trastuzumab that should be investigated in primary mammary cancers.