Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that degrade components of the extracellular matrix (ECM) and play essential roles in various physiological and pathological processes such as morphogenesis, differentiation, angiogenesis, tissue remodeling, and tumor invasion. A typical MMP consists of a propeptide of about 80 amino acids, a catalytic metalloproteinase domain of about 170 amino acids, a linker peptide of variable lengths and a hemopexin domain of about 200 amino acids. The zinc binding motif HEXXHXXGXXH in the catalytic domain, and the ‘‘cysteine switch’’ motif PRCGXPD in the propeptide are common structural signatures. MMP-12 (macrophage elastase) is a 54 kDa proenzyme consisted of propeptide, catalytic metalloproteinase domain, and hemopexin domain. The predicted human 54-kD protein is processed by loss of both N- and C-terminal residues to a 22-kD mature form. It is a major proteinase that degrades elastin. MMP-12 can also degrade a broad spectrum of substrates, including type IV collagen, fibronectin, laminin, vitronectin, proteoglycans, chondroitin sulfate, myelin basic protein, alpha 1-antitrypsin, and plasminogen. MMP-12 is mainly produced by macrophages and has been shown to be associated with inflammatory skin diseases, atherosclerosis, aneurysms, and cancers. MMP12 can exhibit both proinflammatory and anti-inflammatory activity in a tissue- or disease context-dependent manner. The purified recombinant MMP-12 is consisted of pro-peptide and catalytic domain.