Follistatin is a single-chain glycoprotein that inhibits the secretion of FSH from the pituitary, initially isolated from porcine follicular fluid. It is a TGF-β family member antagonist (which also include Noggin, Chordin, DAN/Cerberus) and binds to the ligands to block receptor binding. Follistatin binds to Activin with high affinity (Kd ~45 pM), but has also been reported to bind other ligands, such as BMPs 2, 4, 6, 7, 11, 15, and myostatin, with lower affinity. Follistatin knockout mice display defects in musculoskeletal development, and die shortly after birth. It’s primarily expressed in the liver, ovary, kidney, and the brain. Specifically in the ovary, Follistatin mRNA expression is low in primordial follicles, but increases in the granulosa cells in secondary and tertiary follicles. Dysregulated Follistatin and Activin expression have been associated with several hepatic diseases, such as viral hepatitis, fibrosis, and hepatocellular carcinoma.