ERBB2 and breast cancer: HER2 is overexpressed in approximately 20% of breast cancer patients.57 In 2013, Ado-trastuzumab emtansine (also known as T-DM1 or Kadcyla®, Genentech, Inc.), a HER2-targeting ADC incorporating the anti-HER2 trastuzumab with the microtubule inhibitor DM1 (a maytansine derivative) via a stable thioether linker, was approved for the treatment of patients with HER2-positive metastatic breast cancer.58 The approval was based on a phase III trial (EMILIA).59 In the EMILIA trial, patients were randomly administered either T-DM1 (n = 495) or lapatinib plus capecitabine (n = 496).