Purity >95% as determined by SDS-PAGE. Follistatin is an autocrine, activin-binding protein that is ubiquitously expressed with highest expression levels being in the ovary and skin. Follistatin negatively regulates the signaling of transforming growth factor beta (TGF-?) family members, such as activin, bone morphogenic proteins (BMPs), myostatin, growth differentiation factor 11 (GDF-11), and TGF-?1. Follistatin functions as an antagonist by binding TGF-? family members to block interaction with their signaling receptors. Follistatin also inhibits the secretion of follicle-stimulating hormone (FSH) from the anterior pituitary.