IPI-504 is a novel, water-soluble analog of 17-AAG and a potent inhibitor of Hsp90. Hsp90's role in the cell is to control the proper folding, function, and viability of various "client" proteins. Many of these client proteins (such as AKT, Her-2, Bcr-Abl, PDGFR-α, and c-Kit) are oncoproteins or important cell signaling proteins. In patients with GIST, mutations in the tyrosine kinase receptor Kit play a critical role in the pathogenesis of this disease. Inhibition of Kit signaling with the tyrosine kinase inhibitor Imatinib (IM) is a very effective treatment for GIST patients. However, new mutations arise in Kit conferring resistance to IM treatment which results in disease progression. Kit is a client protein of Hsp90 and is sensitive to IPI-504. In Soft Tissue Sarcomas, there may be genetic abnormalities that lead to the expression of certain proteins that drive the growth of cancer. These cancer-driving proteins may be stimulated by HSP90. This provides a scientific rationale for Phase 1 clinical testing of IPI-504 in patients with advanced GIST and STS who have failed prior therapies.