“…However, the sensitivity of this yeast glucocorticoid assay is relatively very low. This becomes clear when the EC 50 values for E2, P4, T and Dex of respectively 0.5, 1, 50 and 100.000 nM in yeast-based ER, PR, AR and GR bioassays are compared with their U2-OS-based CALUX analogues, with EC 50 values of respectively 0.02, 0.6, 0.7 and 0.7 nM [29, 30, 33, 42–44]. The relatively very low sensitivity of the yeast glucocorticoid bioassay is most probably due to the absence of the human HSP90 chaperone protein; the present data thus suggest that HSP90 is more needed in yeast to obtain optimal hGRα activity than it is needed to obtain fully active ER, PR and AR.…”