ERBB2 and breast cancer: Estimates suggest that HER2-positive breast cancer accounts for 15–20% of all breast cancer cases, and HER2 is an independent prognostic factor.1 Trastuzumab, a humanised monoclonal antibody against HER2, has been reported to markedly improve the prognosis of patients with HER2-positive breast cancer.2,3 Accordingly, trastuzumab is widely used as a standard therapy for HER2-positive breast cancer.