Hsp90 is a member of the heat shock protein 90 family that functions as a molecular chaperone and has ATPase activity. Hsp90 family proteins are highly conserved between isoforms and species. Several signal transduction pathways depend on Hsp90 function including erbB2, steroid hormone receptors (such as androgen, progesterone, glucocorticoid, and aryl-hydrocarbon), and hypoxia sensing (Hif1alpha). Recent reports show that tumor cells are more sensitive to Hsp90 inhibition and that Hsp90 from tumor cells is more sensitive to small molecule inhibitors (eg 17AAG). The mechanism of this differential sensitivity of normal versus tumor Hsp90 is not known (although mutation has been ruled out). One possible mechanism may be differences in post-translational modification of tumor Hsp90. Hsp90 is a cytoplasmic protein that forms a homodimer in vivo, and interacts with TOM34, AHSA1, HDAC6 and SMYD3.