 IPI-504 is a novel small molecule inhibitor of heat shock protein 90 Hsp90, an emerging and recently identified target for cancer therapy. Hsp90 is a protein chaperone that plays a central role in regulating protein homeostasis. Hsp90 regulates the stability of key proteins called client proteins and keeps them in the appropriate three dimensional shape so they can perform their cellular functions. In addition, many of the proteins stabilized by Hsp90 are oncoproteins and cell-signaling proteins important in cancer cell proliferation and cancer cell survival. Thus Hsp90, a single molecular target that is a central integrator of multiple pathways important to cancer, is an ideal novel target for oncologic therapy. Selective inhibition of Hsp90 will affect multiple downstream mechanisms to disrupt tumor growth and selectively kill cancer cells. The anti-neoplastic effects of Hsp90 inhibition have been demonstrated both in vitro and in vivo for a variety of different hematologic and solid tumors including multiple myeloma. 