Follistatin was first identified as a follicle-stimulating hormone inhibiting substance present in ovarian follicular fluid. It has since been shown to be a multifunctional regulatory protein exerting a majority of its effects via neutralization of activin. FS is a monomeric binding protein that regulates activin activity by forming an inactive complex. FS plays a role as a tissue regulator in the gonad, pituitary gland,pregnancy membranes, vasculature, and liver. It is also essential for normal development, as FS-knockout mice die shortly after birth with a range of defects including insufficient muscle development and skeletal abnormalities.FS is a single-chain polypeptide with a structure unlike activin or inhibin. Alternative splicing events, proteolytic cleavage and variable glycosylation lead to a vast array of possible FS isoforms found within various biological fluids.