To interrogate the whereabouts of ErbB2 following HSP90 inhibition, we carried out immunofluorescence assays to investigate the intracellular distribution of ErbB2 in a panel of ErbB2-overexpressing breast cancer cells (AU565, SKBR3, HCC1419, HCC1954, and ZR-75-30) treated with 17-AAG, a widely used HSP90 inhibitor with clinical efficacy [16]. This evidence concerns the gene ERBB2 and breast carcinoma.