ERBB2 and breast cancer: Since the first monoclonal antibody targeting the extracellular domain of HER2, Trastuzumab, got approved by the US Food and Drug Administration (FDA) for the treatment in HER2-positive breast cancer in 1998, a number of anti-HER2 agents have been developed and modified for the treatment of patients harboring ERBB2 amplifications, including monoclonal antibodies, small molecular drugs, and antibody-drug conjugates (ADCs).