Protease-inhibitor that contains multiple distinct protease inhibitor domains. Probably has serine protease- and metalloprotease-inhibitor activity (By similarity). Inhibits the biological activity of mature myostatin, but not activin. Interacts with both mature and propeptide myostatin/MSTN. Widely expressed, with high expression in skeletal muscle and heart. Also expressed in brain, lung and testis. Weakly expressed in liver and kidney. Belongs to the WFIKKN family.