Geldanamycin is a benzoquinone antineoplastic antibiotic isolated from the bacterium Streptomyces hygroscopicus. Geldanamycin binds to and inhibits the cytosolic chaperone functions of heat shock protein 90 (HSP90) (Kd = 1.2 μM). HSP90 maintains the stability and functional shape of many oncogenic signaling proteins.