ERBB2 and breast cancer: Trastuzumab (Herceptin®) is a classical monoclonal (monospecific) antibody (Ab) that binds to the extracellular domain 4 (ECD4) of HER2 and was the first FDA-approved HER2-targeted therapy for the treatment of HER2-overexpressing breast cancer as well as gastric/ gastric esophageal junction (GEJ) adenocarcinoma5.