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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 mRNAFollistatin binds directly to activin and functions as an activin antagonist. specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone (fsh).

Follistatin is being studied for its role in regulation of muscle growth in mice, as an antagonist to myostatin (also known as GDF-8, a TGF superfamily member) which inhibits excessive muscle growth. Lee & McPherron demonstrated that inhibition of GDF-8, either by genetic elimination (knockout mice) or by increasing the amount of follistatin, resulted in greatly increased muscle mass. In 2009, research with macaque monkeys demonstrated that regulating follistatin via gene therapy also resulted in muscle growth and increases in strength.