MMP9, also known as Matrix metallopeptidase 9 or gelatinase B is a member of the matrix metalloproteinase (MMP) family. The MMPs are zinc-containing enzymes involved in the degradation of the extracellular matrix in normal physiological processes including embryonic development, reproduction and tissue remodeling. Most MMP family members are secreted as inactive proproteins that become activated when cleaved by extracellular proteinases. MMP9 is secreted as a 92kDa zymogen and cleavage of pro-MMP9 results in the active enzyme with a molecular weight of 82kDa. MMP9 has a gelatin-binding domain consisting of three fibronectin type II units, a catalytic domain containing the zinc-binding site, a proline-rich type V collagen-homologous domain and a hemopexin-like domain. MMP9 is produced by monocytes, macrophages, neutrophils, keratinocytes, fibroblasts, osteoclasts and endothelial cells, and is involved in inflammatory responses, tissue remodeling, wound healing, tumor growth and metastasis. MMP9 is supplied by bone marrow-derived cells and contributes to skin carcinogenesis. Further, MMP9 degrades type IV and V collagens. Studies in rhesus monkeys suggest that the enzyme is involved in IL-8-induced mobilization of hematopoietic progenitor cells from bone marrow, and murine studies suggest a role in tumor-associated tissue remodeling. Studies have shown that elevated MMP9 is associated with progression of idiopathic atrial fibrillation and aortic aneurysm.