Follistatin is a protein that can increase muscle mass beyond natural potential by suppressing myostatin. Also known as activin-binding protein is a protein that in humans is encoded by the FST gene. Follistatin is an 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. Follistatin is a multidomain, secreted glycoprotein, with a molecular weight in the 31–39 kD range, due to alternative splicing.