The matrix metalloproteinases (MMPs) consist of 24 known human zinc proteases with essential roles in breaking down components of the extracellular matrix (ECM). Additional MMP substrates include cytokines, chemokines, growth factors and binding proteins, cell/cell adhesion molecules, and other proteinases. With a few exceptions, MMPs share common structural motifs including a pro-peptide domain, a catalytic domain, a hinge region, and a hemopexin-like domain. Synthesized as pro-enzymes, most MMPs are secreted before conversion to their active form. MMP activities are modulated on several levels including transcription, pro-enzyme activation, or by their endogenous inhibitors, tissue inhibitors of metalloproteinases (TIMPs). A subset of MMPs are associated with membranes and designated as membrane-type metalloproteinases (MT-MMP).