Protein C inhibitor (PCI, SERPINA5) is a serine protease inhibitor (serpin) which limits the expression of protein C.Suzuki et al. (1987) determined the sequence of cDNA for human protein C inhibitor (PCI). A high degree of homology of the deduced amino acid sequence to that of other known inhibitors clearly demonstrated that protein C inhibitor is a member of the superfamily of serine protease inhibitors including alpha-1-antichymotrypsin (AACT), alpha-1-antitrypsin (AAT; PI), antithrombin III (AT3), and angiotensinogen (AGT).

Protein C inhibitor is sometimes referred to as plasminogen activator inhibitor-3 (PAI3) because it also inhibits plasminogen activators (Meijers and Chung, 1991). Plasminogen activator inhibitor-1 (PAI1) has structural similarities to PCI.