Heat shock protein 90 (Hsp90) is a molecular chaperone required for the activity of many of the most important regulatory proteins of eukaryotic cells (the Hsp90 ‘clients’). Vertebrates have two isoforms of cytosolic Hsp90, Hsp90aand Hsp90b. Hsp90bis expressed constitutively to a high level in most tissues and is generally more abundant than Hsp90a, whereas Hsp90a is stress-inducible and overexpressed in many cancerous cells.