Follistatin also known as activin-binding protein is a protein that in humans is encoded by the FST gene. Follistatin is an autocrine glycoprotein that is expressed in nearly all tissues of higher animals. The activin-binding protein follistatin is produced by folliculostellate (FS) cells of the anterior pituitary. Its primary function is the binding and bioneutralization of members of the TGF-β superfamily. Follistatin is being studied for its role in regulation of muscle growth in mice, as an antagonist to myostatin.
A study has also shown that increased levels of follistatin, by leading to increased muscle mass of certain core muscular groups, can increase life expectancy in cases of spinal muscular atrophy (SMA) in animal models. It is also being investigated for its involvement in polycystic ovary syndrome.