HSP90,encoded by HSP90AA1,is a constitutively and ubiquitously expressed molecular chaperone that is crucial for the stability and function of many proteins. HSP90 provides chaperoning activity for client proteins; many of them are members of oncogenic pathways,indicating its implication in tumor malignancy. HSP90 mainly resides in the cytosol,while it can also be released to the extracellular space. Secreted Hsp90 is a C-terminal truncated form. It has been reported that the level of plasma Hsp90 is positively correlated with tumor malignancy in clinical cancer patients,and can be a promising diagnostic marker for tumor malignancy in clinical application.