Follistatin belongs to a group of structurally-diverse diffusible proteins that binds to TGF-beta ligands and inhibit their activity by hindering access to signaling receptors. This naturally occurring antagonist binds to Activin, BMP-2,-4,-6,-7, Myostatin, GDF-11, and TGF-beta1. Follistatin is expressed in the pituitary, ovaries, decidual cells of the endometrium, and in some other tissues. Human Follistatin is a 31.5 kDa protein containing 288 amino acids.