Matrix metalloproteinase 9 (MMP-9) is a 92 kDa secreted protein, belonging to the metzincin (multi domain zinc (II) dependent endopeptidases) superfamily of proteases. It is produced by normal and transformed cells. MMP-9 functions through enzymatic degradation by cleaving extracellular matrix proteins and adhesion molecules (like ICAM-5). These events play major roles in the processes of synaptic plasticity, learning, memory, and morphological reconstruction of targets such as neuronal dendritic spines. MMP-9 has been shown to be linked to various disease states including cancer, cardiovascular disease and arthritis. Specifically, cancer models have shown directly that metastasis/angiogenesis and overall tumor aggression are linked to elevated MMP-9 levels. Cardiovascular issues including myocardial infarction, aneurysms, and atherosclerotic plaques have been shown to be linked to increased MMP-9 levels using knockout and overexpression studies in mice. Allograft studies of renal transplant patients have unearthed links between MMP-9 and immune-mediated tissue rejection (destruction) of the allograft opening up windows for rejection prediction in future transplant cases.