A broadly distributed protein that binds directly to activins. It functions as an activin antagonist, inhibits follicle stimulating hormone secretion, regulates cell differentiation, and plays an important role in embryogenesis. Follistatin is a single glycosylated polypeptide chain of approximately 37-kDa and is not a member of the inhibin family (inhibins). Follistatin also binds and neutralises many members of the Transforming growth factor beta family.