STAT3 and breast cancer: From the above PCR array (Supplementary Fig. 2A), we found that MUC1 and MUC4, two STAT3 target genes previously implicated as a mediator of trastuzumab resistance in breast cancer [26-31], were most prominently upregulated in trastuzumab-resistant cells with a more preferential upregulation of MUC1 (2.83 fold) and MUC4 (8.49 fold) seen in BT474R and NCI-N87R cells respectively.