MATRIX METALLOPROTEINASE-9 or MMP-9 (EC 3.4.24.35) is a Ca2+-and Zn2+-dependent, interstitial gelatinase that plays a role in the remodeling of the extracellular matrix of tissues. MMP-9 plays a central role in processes diverse as wound healing, angiogenesis, and bone development. In addition to its role in normal growth and development, MMP-9 is up-regulated in many diseases, including metastatic cancer, rheumatoid arthritis, and asthma. BioAssay Systems’ EnzyFluo™ MMP-9 Inhibitor Assay Kit uses a quenched synthetic substrate. Upon digestion by MMP-9, the substrate is cleaved, which abolishes the quenching of the fluorescence label. The fluorescent fragment is measured at λex/em = 490/520 nm. Inhibition is determined by the decrease in fluorescence.