Pimitespib (TAS-116) is a novel, small-molecule HSP90 inhibitor which inhibits geldanamycin-FITC binding to HSP90 proteins with Ki values of 34.7 nmol/L, 21.3 nmol/L, >50,000 nmol/L, and >50,000 nmol/L for HSP90α, HSP90β, GRP94, and TRAP1, respectively. Furthermore, TAS-116 does not inhibit other ATPases such as HSP70 (IC50 >200 μmol/L).