AT13387, discovered by a high-throughput x-ray craystallography fragment-based platform, is a non-geldanamycin inhibitor of heat shock proterin 90 (HPS90), a molecule chaperone regulating signal transduction pathways for cell growth and survival. It has high affinity for binding HSP90 (Kd = 0.5nM) and exhibited an in vitro cytotoxicity showing a median EC50 value of 41 nM. The antitumor activity of AT13387 against solid tumor and leukemia models is similar to alvespimycin.