Human epidermal growth factor 2 (HER2)-positive breast cancers are characterised by amplification of the HER2 gene or overexpression of the HER2 protein and account for 15–20% of breast malignancies.1 The introduction of HER2-targeted therapies has changed the treatment paradigm of HER2-positive cancers, which were previously associated with more aggressive disease and poorer outcomes.2,3. The gene discussed is ERBB2; the disease is cancer.