Trastuzumab has been demonstrated to prolong OS (15.2 versus 9.9 months) and delay the onset of BM in HER2-positive BC, suggesting a preventive role by blocking the entry of tumor cells into the CNS [156]; however, CNS is the first site of relapse following trastuzumab due to the poor ability to cross the BBB [11,157,158]. Here, ERBB2 is linked to breast cancer.