Matrix metalloproteinases are a family of zinc- and calcium-dependant endopeptidases, which degrade extracellular matrix proteins. Matrix metalloproteinase 9 (MMP-9) is secreted as a 92-kDa zymogen (pro-MMP-9) cleaved into the 82-kDa active enzyme. MMP-9 has a gelatin-binding domain consisting of three fibronectin type II units, a catalytic domain containing the zinc-binding site, a proline-rich type V collagen-homologous domain, and a hemopexin-like domain. MMP-9 is produced by monocytes, macrophages, neutrophils, keratinocytes, fibroblasts, osteoclasts and endothelial cells, and is involved in inflammatory responses, tissue remodeling, wound healing, tumor growth, and metastasis. Recombinant human Pro-MMP-9 comprises a 688 aa fragment (20-707) corresponding to the pro form of the protein minus the signal peptide and is expressed in E. coli with an amino-terminal hexahistidine tag.