Tubulysin M is a highly cytotoxic peptide isolated from the myxobacterial species Archangium geophyra and Angiococcus disciformis. Tubulysin displays extremely potent cytotoxic activity in mammalian cells, including multidrug-resistant cell lines, with IC50 values in the lower nanomolar range. Tubulysin M is a cytotoxic activity tubulysin which inhibits tubulin polymerization and leads to cell cycle arrest and apoptosis.