Stromelysin-1 is an enzyme that in humans is encoded by the MMP3 gene. Proteins of the matrix metalloproteinase (MMP) family are involved in the breakdown of extracellular matrix proteins and during tissue remodeling in normal physiological processes, such as embryonic development and reproduction, as well as in disease processes, such as arthritis, and tumour metastasis. Most MMPs are secreted as inactive proproteins which are activated when cleaved by extracellular proteinases. The MMP-3 enzyme degrades collagen types II, III, IV, IX, and X, proteoglycans, fibronectin, laminin, and elastin. In addition, MMP-3 can also activate other MMPs such as MMP-1, MMP-7, and MMP-9, rendering MMP-3 crucial in connective tissue remodeling. The enzyme is also thought to be involved in wound repair, progression of atherosclerosis, and tumor initiation. MMP3 can enter in cellular nuclei and control transcription. MMP-3 has been implicated in exacerbating the effects of traumatic brain injury (TBI) through its disruption of the blood-brain barrier (BBB). MMP-3 also does damage to the blood-spinal cord barrier (BSCB), the functional equivalent of the blood-brain barrier, after spinal cord injury (SCI).