ERBB2 and breast cancer: Treatment for TNBC [tumors that are estrogen receptor (ER)-negative, progesterone receptor (PR)-negative and human epidermal growth factor receptor 2 (HER2) non-amplified] continues to be a challenge due to the fact that it is, by definition, insensitive to the hormonal therapies and trastuzumab that have been developed to treat other types of breast cancer.