ERBB2 and breast cancer: Trastuzumab emtansine (T-DM1) is composed of: 1) trastuzumab, a humanized HER2-targeting mAb that have been approved for the treatment of HER2-positive breast cancer; 2) a non-cleavable thioether linker, N-maleimidomethyl cyclohexane-1-carboxylate (MCC); and 3) a potent microtubule-depolymerizing maytansinoid derivative, DM1 (Corrigan et al., 2014).