Follistatin is a high affinity binding protein of Activin originally isolated for its role in regulating the release of follicle-stimulating hormone (FSH). Follistatin forms a group of interrelated factors with Activins and inhibins, members of the transforming growth factor beta (TGFbeta) superfamily. Activin, follistatin and Activin receptors are expressed in many tissues, where they function as autocrine/paracrine regulators of a variety of physiological processes including reproduction. Follistatin is an important regulator of pituitary FSH secretion.