MMP9 and breast cancer: More than 45 years ago, it was established that thrombocytopenic mice were protected against metastasis.[31] Since then, extensive data has supported the relevance of platelets in the progression of cancer.[32] Janowska-Wieczorek et al. showed that platelet-derived microparticles stimulate mitogen-activated protein kinases in lung carcinoma cell lines and increase cell proliferation.[33] Platelets induce matrix metalloproteinase (MMP)-9 expression and activation in the cell lines of colon and breast cancer.