Hsp90 beta (heat shock protein 90 beta) is a constitutively expressed isoform of Hsp90, one of the most abundant chaperones in the cytosol of eukaryotic cells. Hsp90 interacts with various proteins, including protein kinases and transcription factors, and either facilitates their stabilization and activation or directs them for proteasomal degradation. Hsp90 thus affects multiple signaling pathways and biological processes and modulation of this single target offers the prospect of simultaneous intervence to various key points of oncogenic transformation. Hsp90 operates as a dimer in a conformational cycle driven by ATP binding and hydrolysis.