Geldanamycin, a well-known cell-permeable inhibitor of intracellular Hsp90, which causes a simultaneous degradation of many Hsp90 client proteins followed by the suppression of the growth and killing of cancer cells [32], demonstrated strong cytotoxic and antiproliferative activities, which indicated the validity of the MTT assay in the analysis of the cytotoxic and antiproliferative properties of the polymeric conjugate. This evidence concerns the gene HSP90AA1 and cancer.