PAI-1 is a Mr ~50,000 glycoprotein, which is the primary physiological inhibitor of the two plasminogen activators uPA and tPA. PAI-1 belongs to the serpin protein family. Studies of PAI-1 have contributed significantly to the elucidation of the protease inhibitory mechanism of serpins, which is based on a metastable native state becoming stabilised by insertion of the RCL into the central beta-sheet A and formation of covalent complexes with target proteases. In PAI-1, this insertion can occur in the absence of the protease, resulting in generation of a so-called latent, inactive form of the protein. PAI-1, in its active state, also binds to the extracellular protein vitronectin. When in complex with its target proteases, it binds with high affinity to endocytosis receptors of the low density receptor family.