AT13387 is a synthetic, orally bioavailable, small-molecule inhibitor of heat shock protein 90 (Hsp90) with potential antineoplastic activity. Hsp90 inhibitor AT13387 selectively binds to Hsp90, thereby inhibiting its chaperone function and promoting the degradation of oncogenic signaling proteins involved in tumor cell proliferation and survival. Hsp90, a chaperone protein upregulated in a variety of tumor cells, regulates the folding, stability and degradation of many oncogenic signaling proteins. AT13387 is a selective potent heat shock protein 90 (Hsp90) inhibitor with IC50 of 18 nM in A375 cells. The Kd for AT13387 binding is 0.7 nM.