Hsp90 (90 kDa heat shock protein) is a molecular chaperone that aids protein folding and quality control for a large number of client proteins. Functional Hsp90 operates as dimer and has intrinsic ATPase activity. The Hsp90 dimer acts in concert with other chaperones (e.g. Hsp70) and is regulated by a number of co-chaperones/accessory proteins (e.g. Hop, cdc37). Hsp90 has been shown to interact with > 100 proteins and some notable clients include kinases (e.g. Raf-1), nuclear hormone receptors (e.g. estrogen receptors), transcription factors (e.g. p53), GPCRs (e.g. CB2 receptors) and ion channels (e.g. CFTR). In humans, the Hsp90beta isoform is constitutively expressed whereas the Hsp90alpha isoforms is expressed under stress conditions. Hsp90 plays an important role in some tumor cell types by stabilising mutated oncogenic proteins.