WFIKKN2 is a multidomain extracellular protein that functions as a selective antagonist of myostatin (GDF8) and GDF11 signaling. The protein comprises WAP, follistatin, immunoglobulin, two Kunitz-type protease inhibitor, and netrin domains 1. The follistatin domain primarily mediates binding to mature GDF8/GDF11, while the netrin domain contributes significantly to propeptide interactions 2. WFIKKN2 inhibits GDF8 and GDF11 biological activity in the nanomolar range by blocking type II receptor activation 3, though it binds but does not inhibit TGFβ1, BMP2, and BMP4, suggesting selective modulatory roles 4. In musculoskeletal tissues, WFIKKN2 functions as a "companion" protein regulating localized TGFB presentation and signaling balance 5. Clinical evidence indicates WFIKKN2 expression is sex-dimorphic in pancreatic cancer cachexia, with higher expression in women correlating with reduced muscle wasting, suggesting protective roles in cancer-related myopathy 6. Circulating WFIKKN2 levels do not independently predict grip strength decline in older adults 7. WFIKKN2 expression varies genetically across species, indicating potential associations with growth and muscle traits 8.