Phomopsin B is the dechloro analogue of phomopsin A first reported by Culvenor and colleagues, CSIRO Australia in 1977. Phomopsin B is produced by the fungus Phomopsis leptostromiformis, the causal agent in lupin poisoning (lupinosis). The phomopsins contain a macrocyclic hexapeptide with a 13-membered ring formed by an ether bridge. Phomopsin B is a potent microtubule inhibitor, blocking the polymerization of tubulin at concentrations of less than 1 mM. The phomopsins exert their action on tubulin by interaction at or near the vinblastine binding site.