ERBB2 and breast cancer: HER2 overexpression is a known feature of a subset of breast cancers, with overexpression or activation of the ERBB2 receptor tyrosine kinase (RTK) leading to pro-tumorigenic cell signaling [9] that has been successfully clinically targeted with the monoclonal antibody trastuzumab [10], making it an ideal target for the first ADC for solid tumors.