ERBB2 and cancer: All 14 patients who progressed after first-line treatment with a combination of trastuzumab and/or pertuzumab plus chemotherapy for HER2-positive MBC received a second-line systemic cancer therapy, involving trastuzumab emtansine (n = 6), trastuzumab deruxtecan (n = 4), or a combination of trastuzumab and/or pertuzumab with docetaxel (n = 4) (Table 4 and Figure 4).