These findings show that antibody-drug conjugates (ADC), in particular, trastuzumab deruxtecan [11,12] and trastuzumab duocarmazine [13], may have clinical activity in tumors that have low to moderate levels of HER2 expression beside classical HER2+ BC, revealing objective response rates ranging between 32% and 37% in the HER2-low population [7,11-13]. The gene discussed is ERBB2; the disease is breast cancer.