Heat shock proteins (HSPs) are ubiquitously expressed in all organisms. A major function of HSP90 and other HSPs is to act as molecular chaperones. HSP90 forms a complex with glucocorticoid receptor (GR), rendering the non-ligand-bound receptor transcriptionally inactive. HSP 90 binds the GR as a heterocomplex composed of either HSP56 or cyclophilin-40, forming an aporeceptor complex. HSP90 also exists as a dimer with other proteins such as p60/sti1 and p23, forming an aporeceptor complex with estrogen and androgen receptors.