Expressed in the pituitary, ovaries, and in some other tissues, FST is a high affinity activin binding protein that can act as an activin antagonist. FST was originally discovered through its suppressive effect of the expression and secretion of the pituitary follicle stimulating hormone. Besides activin, FS has also been shown to bind with multiple BMPs, Myostatin, GDF-11, and TGF-beta-1. Human Follistatin contains three cysteine-rich domains (called Follistatin-like domains), 3 Kazal-like domains and one TGF-beta binding domain.