Similar results were obtained by the pharmacological inhibition of HSP90 activity using geldanamycin; although hampered by the strong growth inhibitory effect of geldanamycin alone, pretreatment of HER2+ breast cancer cells with geldanamycin for 24 h significantly impaired further growth inhibition induced by single-agent trastuzumab and docetaxel treatments, as well as growth inhibitory synergism of their combination (Figure S4B). This evidence concerns the gene ERBB2 and breast carcinoma.