T–DM1 is an ADC drug approved in early 2013 for the treatment of human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer that combines the biological activity of HER2 antibody (Herceptin or trastuzumab) with the targeted delivery of a potent antimicrotubule agent mertansine to HER2-expressing breast cancer cells [16,17,18,19,20]. The gene discussed is ERBB2; the disease is breast cancer.