Follistatin, a secreted protein is able to bind and neutralise the actions of many members of the Transforming Growth Factor-beta family of proteins. Follistatin was first implicated in the regulation of follicle-stimulating hormone secretion in the pituitary and subsequently in other regions of the adult body associated with reproductive functions. Recent work has shown that this protein is much more broadly distributed and may also play a significant role during embryogenesis. Gene targetting has shown that follistatin is essential for normal development and in its absence, mice die soon after birth with a range of defects including insufficient muscle development and skeletal abnormalities. A number of diseases have been identified thought to be caused by an over-production of members of the Transforming Growth Factor-beta family of proteins. Therefore it may be possible to use follistatin as a therapeutic agent in these disorders.