ERBB2 and breast cancer: Currently, tucatinib, in combination with trastuzumab and capecitabine, is the only systemic therapy approved by the Food and Drug Administration (FDA) for the treatment of adult patients with advanced unresectable or metastatic HER2+ breast cancer, including patients with BM, who have received one or more prior anti-HER2-based regimens in the metastatic setting [4].