Maytansinol, formerly known as Ansamitocin P-0, is a potent microtubule depolymerizing agent. Maytansinol inhibits microtubule assembly and induces microtubule disassembly in vitro. Maytansinol disrupts the mitotic spindle and prevents mitotic exit in Drosophila. Maytansinol is used in the preparation of site-specific trastuzumab maytansinoid antibody-drug conjugates with improved therapeutic activity.