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 ? (TGF-?) family members, such as activin, bone morphogenic proteins (BMP), 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.