Abstract
Heat Shock Protein 90 (Hsp90) is a ubiquitous molecular chaperone that comprises about 1-3% of the total cellular protein. Over the last decade, Hsp90 has been detected and studied in the extracellular space (extracellular or eHsp90) of normal and neoplastic cells. Once outside the cell, eHsp90 has been shown to interact with extracellular client proteins and promote their stabilization and function. Cell conditioned media are routinely collected to detect and quantify eHsp90, and determine its interactions with extracellular clients. Finally, targeting specifically the eHsp90 with pharmacologic inhibitors or antibodies that are unable to cross the plasma membrane has been beneficial in inhibiting tumor cell motility and invasion.