Complete pathological response (pCR) to neoadjuvant therapy is a recognised surrogate for survival outcomes and, when compared to definite HER2 positive (3+) and HER2 negative (0/1+) groups, provides an indication of the underlying biology of these tumours with respect to the HER2 pathway activation and response to anti-HER2 therapy. This evidence concerns the gene ERBB2 and neoplasm.