Although a number of Stat3 target genes including vascular endothelial growth factor (VEGF), survivin, matrix metalloproteinase-9 (MMP-9) and twist have been identified in primary breast cancers and cancer-derived cell lines, we were interested in identifying additional target genes which may participate in metastatic progression of breast cancer [11], [20], [26], [27], [28], [29]. This evidence concerns the gene MMP9 and breast cancer.