ERBB2 and breast cancer: For example, trastuzumab is only given to the subgroup of breast cancer patients who are human epidermal growth factor receptor 2 (HER‐2) positive, as it is known to lock on to the HER‐2 protein, block the receptor, and stop the cells from dividing and growing.3 It is therefore considered unnecessary for those who are HER‐2 negative.