ERBB2 and breast cancer: In particular, the HSP90 inhibitor, 17-(allylamino)-17-demethoxygeldanamycin (17-AAG), showed therapeutic efficacy in trastuzumab-resistant breast cancer animal models [19] and the HSP90 inhibitor, 17-AAG, combined with trastuzumab also had antitumor activity in trastuzumab refractory HER2 overexpressing breast cancer patients [20].