Serglycin proteoglycans contribute to proper storage and secretion of inflammatory mediators in hematopoietic cells. Serglycin is also expressed in cancer cells where increased expression has been linked to poor prognosis. In the present study we report that serglycin proteoglycan is absolutely required for metastasis in the MMTV-PyMT-driven mouse breast cancer model. Serglycin seems to play a role in promoting epithelial to mesenchymal transition, cancer-related inflammation and extravasation. Our results suggest that serglycin and serglycin-dependent mediators are potential drug targets to prevent metastatic disease/dissemination of cancer. 6 total breast tumor samples were analyzed. 3 of SG+/- and 3 of SG-/- tumour tissue. Raw data was normalized using the robust multi-array average (RMA) method. To identify potential serglycin-regulated mediators of metastasis, we performed a microarray expression analysis of RNA isolated from SG+/- and SG-/- breast tumor tissue. The expression analysis identified 672 genes with a significantly altered expression level, at log2 fold >±1,2. Strikingly, only six genes were up-regulated in the SG-/- PyMT+ tumor cells compared to SG+/- PyMT+ tumor cells while 666 were significantly down-regulated.