ERBB2 and breast cancer: In 2020, 7.8 million women were alive with a diagnosis of breast cancer in the previous 5 years, making breast cancer the most prevalent cancer globally.1 The gene encoding human epidermal growth factor receptor 2 (HER2) is overexpressed in 15–20% of breast cancers and was originally associated with a poor prognosis due to accelerated tumour growth and early metastasis.2 3 Development of therapies to inhibit the HER2 signalling pathway, including the monoclonal antibody trastuzumab, has transformed the outlook of HER2-positive breast cancer.4