ERBB2 and breast cancer: However, since a phase 1 study showed promising clinical antitumor activity and a controlled safety profile in HER2-negative breast cancer patients [55], more clinical studies have found significant efficacy of trastuzumab deruxtecan in HER2-low (IHC 1+ vs. IHC 2+/ISH−) mBC with significantly longer PFS and OS than the physician’s choice of chemotherapy (10.1 months versus 5.4 months, 23.9 months versus 17.5 months, respectively) [56].