DS-8201a, an antibody-drug conjugate (ADC) of trastuzumab and duroxtecan, a topoisomerase I inhibitor, was tested in a multiple cohorts phase 1 trial including HER2-positive (N = 111) and HER2-low (immunohistochemistry 1+ or 2+ and in situ hybridization negative; N = 34) breast cancer patients [16]. This evidence concerns the gene ERBB2 and breast cancer.