Matrix metallopeptidase 9 (MMP-9) is a matrixin, a class of enzymes that belong to the zinc-metalloproteinases family involved in the degradation of the extracellular matrix. In humans the MMP9 gene encodes for a signal peptide, a propeptide, a catalytic domain with inserted three repeats of fibronectin type II domain followed by a C-terminal hemopexin-like domain.

MMP9 has been found to be associated with numerous pathological processes, including cancer, immunologic and cardiovascular diseases. One of MMP9’s most widely-associated pathologies is the relationship to cancer, due to its role in extracellular matrix remodeling and angiogenesis. For example, its increased expression was seen in a metastatic mammary cancer cell line. In addition, MMP9 levels increase with the progression of idiopathic atrial fibrillation.