An oncogenic isoform of HER2 associated with locally disseminated breast cancer and trastuzumab resistance.

The HER2-targeted therapy trastuzumab is widely used for the treatment of patients with metastatic breast tumors overexpressing HER2. However, an objective response is observed in only 12% to 24% of patients treated with trastuzumab as a single agent and initial responders regress in <6 months (1-3). The reason for the clinical failure of trastuzumab in this setting remains unclear.