WFIKKN1 is a multidomain extracellular protein containing WAP, follistatin, immunoglobulin, Kunitz-type protease inhibitor, and NTR domains that functions as a growth factor antagonist and binding protein 1. The protein binds with high affinity to growth and differentiation factors GDF8 (myostatin) and GDF11, serving as a potent antagonist that inhibits their biological activity in the nanomolar range 23. Structure-function studies reveal that the follistatin domain primarily mediates binding to mature growth factors, while the NTR domain contributes significantly to interactions with myostatin propeptide 2. WFIKKN1 also binds TGFβ1, BMP2, and BMP4 with high affinity but does not inhibit their signaling activity, suggesting it functions as a growth factor binding protein for these factors, potentially localizing their action and establishing growth factor gradients 3. The protein enhances BMP1-mediated activation of latent myostatin, likely by facilitating conformational changes that make cleavage sites more accessible 4. WFIKKN1 appears to function as a 'companion' protein that regulates TGFB activity by providing localized and sustained presentation of growth factors to their receptors 5. In zebrafish, WFIKKN1 is involved in AHR signaling and influences transcriptome, proteome, and behavioral development 6.