– WAP, kazal, immunoglobulin, kunitz and NTR domain-containing protein 2, also known as Growth and differentiation factor-associated serum protein 1, WAP, follistatin, immunoglobulin, kunitz and NTR domain-containing-related protein, WFIKKN-related protein, WFIKKN2 and GASP1, is a secreted protein that belongs to the WFIKKN family. WFIKKN2 contains two BPTI/Kunitz inhibitor domains, one Ig-like C2-type (immunoglobulin-like) domain, one Kazal-like domain, one NTR domain and one WAP domain. WFIKKN2 is primarily expressed in ovary, testis and brain, but not in liver. In fetal tissues, it is primarily expressed in brain, skeletal muscle, thymus and kidney. WFIKKN2 is a protease-inhibitor that contains multiple distinct protease inhibitor domains. It probably has serine protease- and metalloprotease-inhibitor activity. It inhibits the biological activity of mature myostatin, but not activin. WFIKKN2 protein binds mature GDF8/myostatin and myostatin propeptide and inhibits the biological activity of myostatin.