Phomopsin A is a an acidic 13-membered cyclic hexapeptide-like metabolite with three unusual amino acids linked in an ‘ansa’ macrocycle with a tripeptide tail, terminating in a dicarboxylic acid. Phomopsin A is a potent mycotoxin produced by the fungus, Phomopsis leptostromiformis, and causes lupinosis in livestock fed infected lupins. Phomopsin A is an important bioprobe for understanding cellular structural proteins. It binds selectively to dimeric tubulin at a site overlapping that of vinblastine and maytansine, inhibiting the formation of the microtubule spindle to block cell division. Uniquely, phomopsin A protects tubulin from decay.