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Serpin A4, also known as Kallistatin or KBP (kallikrein-binding protein), is a serine protease inhibitor (serpin), which can specifically bind to tissue kininogenase and block its activity. However, Serpin A4 itself not only has endothelium-independent vasodilator activity, but also competes with VEGF for receptors and down-regulates the expression of VEGF.