ERBB2 and breast cancer: A recent case-matched analysis of 700 breast cancers [45], stratified by HER2 tumour positivity and BRCA mutation status, showed significant clinical and pathological difference between HER2-positive BRCA1/2mut and HER2-negative BRCA1/2mut tumours, with a higher proliferative index (Ki67) and grade in the HER2-positive cancers.