Noggin is a secreted homodimeric glycoprotein that binds to ligands of the TGF-beta family (BMPs) and regulates their activity by inhibiting their access to signaling receptors. Mature human Noggin protein contains an N-terminal acidic region, a central basic heparin-binding segment and a C-terminal cysteine knot structure. Noggin binds different BMPs with variable affinities, antagonizing specific BMPs during skeletal development. Noggin is expressed in defined areas of the adult central nervous system and peripheral tissues such as lung, skeletal muscle and skin.