ERBB2 and breast cancer: Following trastuzumab, three HER2‐targeted drugs were developed to treat HER2‐overexpressing breast cancer: lapatinib, a small‐molecule dual tyrosine kinase inhibitor that targets EGFR and HER2; trastuzumab DM1 conjugate (also known as T‐DM1), an antibody‐drug conjugate consisting of trastuzumab, a thioether ligand, and derivatives of an antimitogenic drug (maytansine); and pertuzumab, a monoclonal antibody that binds to subdomain II of the HER2 extracellular domain, thereby preventing HER2 from homologous or heterologous dimerization with other HER family receptors.256, 257, 258