The rare capacity for heat shock protein 90 (Hsp90) chaperones to support almost the entire cellular signaling network was viewed as a potential breakthrough to combat tumor resistance to single-oncogene-based therapeutics. Over 2 decades, several generations of Hsp90 ATP binding inhibitors have entered numerous cancer clinical trials, but few have advanced to FDA approval for treatment of human cancers. Herein, we report that Hsp90 expression varies dramatically, especially among different types of noncancer cells and organs. The highly variable levels of Hsp90, from as low as 1.7% to as high as 9% of their total cellular proteins, were responsible for either an extreme sensitivity or an extreme resistance to a classical Hsp90 ATP-binding inhibitor. Among randomly selected cancer cell lines, the same client proteins for regulation of cell growth exhibited unexpectedly heterogenous reactions in response to an Hsp90 ATP-binding inhibitor, inconsistent with the current understanding. Finally, a minimum amount (<10%) of Hsp90β was still required for client protein stability and cell survival even in the presence of full Hsp90α. These new findings of Hsp90 expression in host and isoform compensation in tumor cells could complicate biomarker selection, toxicity readout, and clinical efficacy of Hsp90-ATP-binding inhibitors in cancer clinical trials.