ERBB2 and neoplasm: For all samples, Haematoxylin‐and‐Eosin (H&E) staining as well as Immunohistochemistry (IHC) for ERα were performed to enable assessment of tumour percentage (ranging between 35% and 90% in all samples) and status of ERα, progesterone receptor (PR) and HER2 (Table S1).