N2'-Deacetyl-N2'-(3-mercapto-1-oxopropyl)maytansine, commonly referred to as DM1, is a potent cytotoxic maytansinoid derivative. It is a thiol-containing analog of maytansine, a natural product originally isolated from the Ethiopian shrub *Maytenus serrata*. DM1 acts as a microtubule-targeting agent by binding to tubulin, thereby inhibiting microtubule assembly and disrupting mitosis, which leads to cell cycle arrest and apoptosis in dividing cells. The primary and most significant application of DM1 is as a highly potent cytotoxic payload (or warhead) in Antibody-Drug Conjugates (ADCs). Its thiol group allows for site-specific conjugation to monoclonal antibodies via linkers, typically using maleimide chemistry. DM1 is the cytotoxic component of the FDA-approved ADC trastuzumab emtansine (Kadcyla®), used for the treatment of HER2-positive metastatic breast cancer. Its extreme potency (activity in the picomolar range) makes it suitable for targeted delivery via antibodies to minimize systemic toxicity. As a payload, DM1 is a key tool in oncology drug development, enabling the creation of targeted therapies. It is not used as a standalone chemotherapeutic agent due to its high systemic toxicity. Handling requires strict safety protocols in a controlled laboratory environment, as it is a hazardous compound with significant health risks.