Follistatin is a single-chain gonadal protein that specifically inhibits follicle-stimulating hormone release. The single FST gene encodes two isoforms, FST317 and FST344 containing 317 and 344 amino acids respectively, resulting from alternative splicing of the precursor mRNA. In a study in which 37 candidate genes were tested for linkage and association with polycystic ovary syndrome (PCOS) or hyperandrogenemia in 15 families, evidence was found for linkage between PCOS and follistatin. follistatin are expressed and subserve local regulatory roles in numerous extragonadal tissues, including brain, adrenal, bone marrow, and placenta but perhaps most notably in anterior pituitary-the classical target tissue for inhibin, the activin-follistatin system may play a key role in early embryogenesis. Follistatin binds directly to activin and functions as an activin antagonist. Specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone follistatin is a binding protein to activin. Since activin binds to follistatin, it is imperative to determine the nature of the activin/follistatin binding complex.