Follistatin, also known as Activin-binding protein and FST344, is a secreted protein. Follistatin / FST is an autocrine glycoprotein that is expressed in nearly all tissues of higher animals.
Follistatin / FST contains 3 follistatin-like domains, 3 Kazal-like domains and 1 TB (TGF-beta binding) domain. Follistatin / FST is produced by folliculostellate (FS) cells of the anterior pituitary. FS cells make numerous contacts with the classical endocrine cells of the anterior pituitary including gonadotrophs. Isoform 1 of Follistatin / FST is the predominant isoform in serum but is undetectable in follicular fluid.