Matrix metallopeptidase 12 (MMP-12) is able to degrade elastin, entactin, laminin-1, fibronectin, type IV collagen as well as insulin B-chain and casein. MMP-12 is structurally similar to the classical MMPs (MMP-1, MMP-3); it contains a propeptide with autoinhibitory cysteine switch site, a well conserved zinc site, hinge region and hemopexin domain. MMP-12 lacks a transmembrane domain and furin cleavage site. The zymogen for MMP-12 is about 54 kDa, and is quickly activated to the 45 kDa form, and this breaks down to cascade of active forms, ending with the common 22 kDa form.Stimulated macrophages produce MMP-12; it has also been found in osteosarcoma cells, synovial fibroblasts and lung fibroblasts. MMP-12 levels in quiescent cells and tissues are minimal (with the exception of macrophages), and mitogen stimulation or protein concentration is often needed to visualize the bands. In addition, cell types differ greatly in the quantity of MMP-12 produced.