MMP9 (Gelatinase B) is a matrix metalloproteinase that degrades type IV and V collagens. It is involved in local proteolysis of the extracellular matrix, leukocyte migration, mobilizing hematopoietic progenitor cells, bone resorption, wound repair, angiogenesis, and may play a role in tumor-associated tissue remodeling. MMP9 sees higher expression with progression of atrial fibrillation, and also in invasive endometroid ovarian and endometrial cancers. In immunohistochemistry of normal tissue, MMP9 has nuclear and cytoplasmic positivity in hematopoietic cells, non-germinal center cells of the tonsil and lymph node, peripheral leukocytes, and the spleen. References: Critical Reviews in Biochemistry and Molecular Biology. 2013. 48 (3): 222–72, PMID: 23547785; Critical Reviews in Biochemistry and Molecular Biology. 2002. 37 (6): 375–536, PMID: 12540195; The Journal of International Medical Research. 2014. 42 (1): 224–30, PMID: 24345823; Hum Pathol 2011;42:57, PMID: 20970160;