Amplification of the HER2 gene (also known as ERBB2) is present in 10–20% of tumours in patients with early-stage breast cancer and is associated with aggressive cancers and an increased risk of disease recurrence.1, 2 Trastuzumab, a humanised IgG1 monoclonal antibody that targets the extracellular domain of the HER2 protein, improves progression-free survival and overall survival when administered in combination with chemotherapy in HER2-positive metastatic breast cancer.3 Here, ERBB2 is linked to breast cancer.