Serpin E1, also known as plasminogen activator inhibitor-1 (PAI-1), is mainly produced by the endothelium (cells lining blood vessels), but is also secreted by other tissue types, such as adipose tissue. It is a serine protease inhibitor (serpin) that functions as the principal inhibitor of tissue plasminogen activator (tPA) and urokinase (uPA), the activators of plasminogen and hence fibrinolysis. PAI-1 is present in increased levels in various disease states (such as a number of forms of cancer), as well as in obesity and the metabolic syndrome. It has been linked to the increased occurrence of thrombosis in patients with these conditions.