Follistatin is also known as activin-binding protein, FST and FSH-suppressing protein (FSP), and is an secreted autocrine glycoprotein that is expressed in nearly all tissues of higher animals. Its primary function is the binding and bioneutralization of members of the TGF-β superfamily, with a particular focus on activin, a paracrine hormone. Currently there are three reported isoforms, FS-288, FS-300, and FS-315. Two, FS-288 and FS-315, are known to be created by alternative splicing of the primary mRNA transcript. FS-300 (porcine follistatin) is thought to be the product of posttranslational modification via truncation of the C-terminal domain from the primary amino-acid chain. The activin-binding protein follistatin 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. In the tissues activin has a strong role in cellular proliferation. In the blood, activin and follistatin are both known to be involved in the inflammatory response following tissue injury or pathogenic incursion. Follistatin is involved in the development of the embryo. It has inhibitory action on bone morphogenic proteins (BMPs); BMPs induce the ectoderm to become epidermal ectoderm. Inhibition of BMPs allows neuroectoderm to arise from ectoderm, a process which eventually forms the neural plate. Other inhibitors involved in this process are noggin and chordin. Follistatin and BMPs are also known to play a role in folliculogenesis within the ovary. 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.