ERBB2 and breast cancer: HER2-positive (HER2 over-expression of the receptor) is 15–25% of breast cancers.1–3 If untreated, they have a worse prognosis than HER2 negative tumours.4 Within the adjuvant treatment of patients with HER2-positive breast cancer, a therapy with trastuzumab improves the survival, which is confirmed by the four major international studies: Herceptin Adjuvant Trial (HERA), National Surgical Adjuvant Breast and Bowel Project (NSABP) B-31, North Central Cancer Treatment Group (NCCTG) N9831 and BCIRG Breast Cancer International Research Group (BCIRG) 006th.