The large difference in the affinity of geldanamycin for cytosolic Hsp90 in cancer cells (low nanomolar range) and either Grp94 or Trap1 may explain why a biphasic dose response is clearly seen with this compound in MTT assays; for compounds where the affinities are relatively close, this would not be observed as readily. The gene discussed is HSP90AA1; the disease is cancer.