Breast cancer is the most common malignancy in women and the second most common overall, causing ~600,000 deaths annually worldwide.1 Breast cancers are classified clinically into different molecular subtypes,2 based mainly on expression of oestrogen receptor, progesterone receptor and HER2, and this classification defines suitable therapeutic options, including agents targeting oestrogen or HER2 function. This evidence concerns the gene ERBB2 and breast cancer.