WFIKKN proteins are secreted multidomain proteins containing a WAP (whey acidic protein)-, a follistatin/Kazal-, an immunoglobulin-, two Kunitz-type protease inhibitor-domains, and an NTR (netrin) domain. They have been reported to inhibit the activity of trypsin, and to bind GDF11 (growth and differentiation factor 11) and myostatin (GDF8) []. Due to the presence of multiple domains that frequently serve as protease inhibitors, WFIKKN proteins are presumed to control the action of diverse serine proteases as well as metalloproteinases [].In humans, WFIKKN1 expression is observed in pancreas, liver, thymus, kidney, testis and lung and WFIKKN2 (also known as WFIKKNRP) expression is observed in ovary, testis, pancreas, brain and lung [].
