Serpin A5, also well known as protein C inhibitor (PCI), is a member of the human serpin superfamily consists of at least 35 members. It is synthesized in the liver and has been detected in saliva, cerebral spinal fluid, amniotic fluid, tears and semen. As a potent inhibitor of the protein C anticoagulant pathway at the levels of both zymogen activation and enzyme inhibition, serpinA5 additionally inhibits a variety of serine protease including thrombin, factor Xa, several kallekreins and acrosin, and plays a role in the processes of blood coagulation and fertilization. Serpin A5 also inhibits urinary plasminogen activator (uPA), a mediator of tumor cell invasion, and regulates tumor growth and metastasis by inhibiting angiogenesis. Furthermore, recent studies have identified PCI as a potent and direct inhibitor of activated HGFA (hepatocyte growth factor activator), suggesting a novel function in the regulation of tissue repair and regeneration. Similar to serpins C1 and D1, the thrombin inhibitory activity of serpinA5 is enhanced by heparin.