Mertansine (DM1) is primarily used as a potent cytotoxic agent in antibody-drug conjugates (ADCs) for targeted cancer therapy. It is conjugated to monoclonal antibodies that specifically recognize and bind to antigens on the surface of cancer cells. Once the ADC binds to the target cell, it is internalized, and DM1 is released, disrupting microtubule assembly and leading to cell cycle arrest and apoptosis. This targeted approach minimizes damage to healthy cells, making it effective in treating various cancers, including breast cancer and other solid tumors. Its application is particularly notable in drugs like trastuzumab emtansine (T-DM1), used for HER2-positive breast cancer.