ERBB2 and breast cancer: Trastuzumab‐emtansine (T‐DM1), a HER2‐targeting drug, was the first approved ADC by the FDA in 2013, after the results of the EMILIA trial, which demonstrated a progression‐free survival (PFS)1 and overall survival (OS)2 improvement in patients with advanced HER2‐positive breast cancer pretreated with trastuzumab and a taxane.