Negatively regulates bone density. Antagonizes the ability of certain osteogenic BMPs to induce osteoprogenitor differentitation and ossification (By similarity). Homodimer; disulfide-linked. Expressed in embryonic calvaria and femur, and in adult aorta, costa, femur, kidney, lung, ovary, spleen and trachea (PubMed:8605043). Widely expressed throughout the adult central nervous system, including most neurons and their axons (PubMed:27130896). Expression increases in mesenchymal cells at fracture sites during healing. Also highly expressed in chondrocytes and osteoblasts at newly formed cartilage and bone. Belongs to the TGF-beta family.