ERBB2 and metastatic neoplasm: For patients with de novo HER2+ (HR±) metastatic breast cancer who have not received prior HER2-directed therapy or chemotherapy for metastatic disease or with disease that relapses >6 months after completion of (neo)adjuvant chemotherapy + HER2-directed therapy, the standard of care is trastuzumab + pertuzumab + taxane chemotherapy followed by trastuzumab + pertuzumab +/− ET maintenance therapy (strong recommendation).