Trastuzumab emtansine, also known as ado-trastuzumab emtansine, is an antibody-drug conjugate consisting of the monoclonal antibody trastuzumab (Herceptin) linked to the cytotoxic agent emtansine (DM1). Trastuzumab alone stops growth of cancer cells by binding to the HER2/neu receptor, whereas DM1 enters cells and destroys them by binding to tubulin. Trastuzumab binding to Her2 prevents homodimerization or heterodimerization (Her2/Her3) of the receptor, ultimately inhibiting the activation of MAPK and PI3K/AKT cellular signalling pathways. Because the monoclonal antibody targets HER2, and HER2 is only over-expressed in cancer cells, the conjugate delivers the toxin specifically to tumor cells.