Hsp90 inhibition sensitizes tumor cells to other anticancer therapies. The in vitro anti-MM activities of doxorubicin (Doxo, 50 ng/mL, 48 hours, A) and bortezomib (PS-341, 5 nM, 24 hours, B) are enhanced by 17-AAG treatment (0.75 μM for the final 24 hours of each incubation) in primary MM tumor cells that are resistant to cytotoxic chemotherapy and bortezomib. Error bars indicate SD.