Matrix metalloproteinase 9(MMP-9), also known as 92Kdaiv-type collagenase, 92kDa gelatinase, or gelatinase B, is an enzyme belonging to the zinc-metalloproteinase family. MMP-9 is involved in extracellular matrix degradation in normal physiological processes (such as embryonic development, reproduction, angiogenesis, bone development, wound callus migration, learning and memory) and pathological processes (such as arthritis, cerebral hemorrhage, and tumor metastasis). MMP-9 and elastase appear to be regulators of neutrophil migration through the basement membrane. MMP-9 plays an important role in blood vessel and neovascularization. It is significantly upregulated during epithelial healing of the human respiratory tract and has been found to be associated with many pathological processes, such as cancer, immune and cardiovascular diseases. Elevated levels of MMP-9 were found in cases of rheumatoid arthritis and focal cerebral ischemia. Its level increases with the development of idiopathic AF.