Serine protease inhibitor selective for kallikreins. Efficiently inhibits KLK4, KLK5, KLK6, KLK7, KLK12, KLK13 and KLK14. Doesn't inhibit KLK8. Inhibits acrosin, trypsin, and chymotrypsin. Seminal plasma. The sequence of BUSI-IIA is shown.