A synthetic, small molecule Hsp90 inhibitor that binds competitively with geldanamycin in the ATP-binding pocket of Hsp90. In tumor cells, BIIB021 induces the degradation of Hsp90 client proteins including HER-2, AKT, and Raf-1 and up-regulates expression of the heat shock proteins Hsp70 and Hsp27. BIIB021 inhibits the proliferation of N87, MCF-7, and BT474 tumor cells with IC₅₀ values of 0.06, 0.31, and 0.14 µM, respectively.