ERBB2 and breast cancer: A lysine conjugated ADC comprised of Trastuzumab and the maytansinoid drug payload, N(2′)-deacetyl-N(2′)-(3-mercapto-1- oxopropyl)-maytansine (DM1), which is also known as Ado-Trastuzumab Emtansine (T-DM1), has recently been approved for the treatment of Her2 positive breast cancer patients [8].