Serpina 4，also called Kallistatin, PI4 or TISSUE KALLIKREIN INHIBITOR, is a protein that in humans is encoded by the SERPINA4 gene.The serpina4 is a member of the serpin superfamily and represents a major inhibitor of human tissue kallikrein in the circulation. The SERPINA4 gene is localized to 14q32.13. The translated amino acid sequence of serpina4 matches the protein sequence and shares 44 to 46% sequence identity with other serpin family members. Kallistatin is a serine proteinase inhibitor (serpin) and a heparin-binding protein. It is localized in vascular smooth muscle cells and endothelial cells of blood vessels, suggesting that it may be involved in the regulation of vascular function. And serpina4 also plays a role in neointima hyperplasia.