MMP9 is an extracellular protease, which can be released from neurons, glia, and leukocytes after various types of insults.14 MMP9 is also a major contributor to inflammatory responses after brain insults; it increases BBB permeability and enables leukocyte migration into the brain.14,59 MMP9 has also been involved in the pathogenesis of several autoimmune disorders.60 Importantly, chronically elevated levels of MMP9 (e.g., after trauma or stroke) have been implicated in neurodegeneration.14 Here, MMP9 is linked to Stroke.