HSP90 proteins are highly conserved molecular chaperones that have key roles in signal transduction, protein folding, protein degradation, and morphologic evolution. HSP90 proteins normally associate with other cochaperones and play important roles in folding newly synthesized proteins or stabilizing and refolding denatured proteins after stress. There are 2 major cytosolic HSP90 proteins, HSP90AA1, an inducible form, and HSP90AB1 (MIM 140572), a constitutive form. Other HSP90 proteins are found in endoplasmic reticulum (HSP90B1; MIM 191175) and mitochondria (TRAP1; MIM 606219) (Chen et al., 2005 [PubMed 16269234]).