Hsp90 inhibition targets IGF-1R and IL-6R signaling. Flow cytometric analysis of 17-AAG-treated (750 nM for 24 hours) MM-1S cells indicates complete abrogation of cell surface expression of IGF-1R (CD221) and suppression of surface IL-6R (CD126). The lack of significant suppression of cell surface expression of CD40, another receptor implicated in regulation of MM cell pathophysiology, supports the notion of specificity of the role of hsp90 on surface expression and function of certain growth/survival receptors. Filled curves correspond to staining with anti-IGF-1R, anti-IL-6R, or anti-CD40 antibody; open curves, staining with the respective isotype control antibody.