Members of this family are inhibitors of trypsin family proteases. This domain is highly disulphide bonded. The domain is also found in some large extracellular proteins in multiple copies.

1: A new structural class of serine protease inhibitors revealed by the structure of the hirustasin-kallikrein complex. Mittl PR, Di Marco S, Fendrich G, Pohlig G, Heim J, Sommerhoff C, Fritz H, Priestle JP, Grutter MG; Structure 1997;5:253-264. PMID:9032072