To examine if HSP90 inhibition potentiates the antitumor efficacy of Trast-NG/DOX in vivo, we treated mice bearing xenografts of ErbB2-overexpressing BT-474 or ErbB2-low MCF-7 breast cancer cell lines with Trast-NGs/DOX with or without 17AAG. This evidence concerns the gene ERBB2 and breast cancer.