The fact that matrix metallopeptidase 9 (MMP9) levels are significantly elevated in the course of many neurological diseases has been demonstrated repeatedly, and postmortem studies have observed higher levels of MMP9 in various brain tissues in patients with AD than in cognitively healthy patients, for example, in cytoplasmic neurofibrillary tangles, amyloid plaques, and blood vessel walls in the hippocampus and cerebral cortex [38]. Here, MMP9 is linked to Alzheimer disease.