Geninthiocin, a thiopeptide antibiotic isolated from a Streptomyces sp., is a potent activator of thetipA gene, a bacterial transcription regulator involved in multidrug resistance. Geninthiocin displays a 10-fold selectivity for bacteria over mammalian cells lines, whereas the related dedialanyl geninthiocin shows the reverse selectivity. Geninthiocin is closely related to siomycin, a recently discovered inhibitor of oncogenic transcription factor, FoxM1.