Follistatin is a secreted protein that binds to ligands of the TGF-β family and regulates their activity by inhibiting their access to signaling receptors. It was originally discovered as activin antagonists whose activity suppresses expression and secretion of the pituitary hormone FSH (follicle stimulating hormone). In addition to being a natural antagonist, follistatin can inhibit the activity of other TGF-β ligands including BMP-2,-4,-6,-7, Myostatin, GDF-11, and TGF-β1. Follistatin is expressed in the pituitary, ovaries, decidual cells of the endometrium, and in some other tissues. Human Follistatin is aa 31-33 kDa protein containing 288 amino acids. Its primary structure contains three cysteine-rich domains (called FS domains), each followed by a protease-inhibitory kazal domain.