A 37-year-old female patient presented in September 2012 with HER2+ amplified (immunohistochemistry 3+) stage IV (T1N3M1) cancer of the right breast, with liver, bone, and lymph node metastases...We report here, to our knowledge, the first demonstration of clinical proof of concept for MP0274, with a sustained clinical response in a young patient with metastatic breast cancer treated at a dose level that achieved the high and sustained MP0274 concentrations predicted for required potential efficacy, after failure on two prior lines of therapy, both including an anti-HER2 monoclonal antibody.