Ganetespib (also known as STA-9090) is a potent small-molecule inhibitor of heat shock protein 90 (IC50=4 nM). It binds to the ATP pocket in the N-terminus of Hsp90, resulting in down-regulation of Hsp90 client protein levels. Being structurally unrelated to geldanamycin-derived Hsp90 inhibitors (17-AAG, 17-DMAG, and IPI-504), ganetespib has a unique triazolone-containing chemical structure and stands out other Hsp90 inhibitors in terms of potency, antitumor activity, and safety profile.