ERBB2 and breast cancer: This revision includes clinical question (CQ) 34: “Is trastuzumab deruxtecan recommended for patients with unresectable or metastatic HER2-low breast cancer who have previously received chemotherapy for metastatic disease?”; and future research question (FRQ) 7: “How is HER2-low breast cancer diagnosed for the indication of trastuzumab deruxtecan?”.