In normal, unstressed cells, the chaperone Hsp90 is bound to its cellular client proteins to promote their correct folding and cell homeostasis. Although Hsp90 is expressed in all cells and tissues, a specific tumor-enriched form of Hsp90 (teHsp90) is found in cancer cells. This oncoprotein-bound form appears as a higher-order multichaperone complex with high affinity for certain Hsp90 inhibitors such as PU-H71.