Maytansine is an ansamycin antibiotic originally isolated from the Ethiopian shrub Maytenus serrata. Maytansine inhibits the assembly of microtubules by binding to tubulin at or near the vinblastine-binding site, decreases microtubule dynamic instability and cause mitotic arrest in cells. It exerts cytotoxicity against a number of tumor cell lines and inhibits tumor growth in vivo. However, in human clinical trials, maytansine showed a small therapeutic window due to its neurotoxicity and harmful effects on the gastrointestinal tract. The potent cell killing ability of maytansine can be used in a targeted delivery approach, such as an antibody-drug conjugate, for the selective delivery of the drug and destruction of cancer cells.