Follistatin (FST) is a secreted glycoprotein that binds and neutralizes several TGF-beta superfamily proteins including Activin, BMP-6, BMP-7, and GDF-8/Myostatin. It also binds Angiogenin and regulates hematopoietic stem cell adhesion to Fibronectin. Some Follistatin binding partners will also bind Follistatin-like proteins such as FSL-3. Of three follistatin isoforms, the full-length mature follistatin (FST315) is the most abundant and the sole form in plasma, but has lower binding affinity for both activins and heparins than alternative forms. The acidic tail is missing in the splice variant FST288, which shows highest affinity, and partial in the proteolytically produced FST303, which shows intermediate affinity. Genetic deletion of follistatin in mice, or expression of only the FST288 form, is lethal perinatally due to defects of lung, skin and musculoskeletal system. Expression of only the FST315 isoform allows survival, with defects in vascularization and female fertility.