Purity >98%. Geldanamycin is a benzoquinone ansamycin antibiotic which binds heat shock protein 90 (Hsp90) and its paralog GRP94, altering their actions. Through its interaction with these chaperones, geldanamycin indirectly affects numerous client proteins with roles in diverse cellular processes, including gene expression, cell proliferation, apoptosis, and angiogenesis. For example, geldanamycin inhibits c-jun expression in human colon adenocarcinoma HT29 cells (IC50 = 75 nM), inhibiting AP-1 binding. It also reduces signaling through mitogenic signaling proteins and interferes with steroid receptor function. Inhibitors of Hsp90, including geldanamycin, show promise in cancer therapy.