These proteins inhibit trypsin and chymotrypsin, having 2 sites of interaction with trypsin. The site of interaction with chymotrypsin has not been determined but is not independent of the trypsin-reactive sites. Four inhibitors were found that are identical except at their amino ends and that probably arise by proteolytic degradation of a single gene product. Belongs to the Bowman-Birk serine protease inhibitor family.