ERBB2 and breast carcinoma: PIK3CA mutations protect against the development of malignancy in early breast cancer, inhibiting the proliferation of HER2+ cells by the negative feedback of RTK expression and activation; however, PIK3CA mutations mediated a lower ORR in the trastuzumab‐based neoadjuvant or advanced breast cancers, and enriched in metastatic HER2+ tumours.