Human BMP-2 is one of at least 15 structurally and functionally related BMPs, which are members of the transforming growth factor β (TGF-β) superfamily. BMPs were originally identified as protein regulators of cartilage and bone formation. However, they have since been shown to be involved in embryogenesis and morphogenesis of various tissues and organs. BMPs have also been shown to regulate the growth, differentiation, chemotaxis and apoptosis of various cell types, including mesenchymal cells, epithelial cells, hematopoietic cells and neuronal cells. Similarly to other TGF-β family proteins, BMPs are highly conserved across animal species. At the amino acid sequence level, mature human, mouse and rat BMP-2 are 100% identical. BMP-2 is synthesized as a large precursor protein that is cleaved at the dibasic cleavage site (RXXR) to release the carboxy-terminal domain. Biologically active BMP-2 is a disulfide-linked homodimer of the carboxy-terminal 114 amino acid residues that contains the characteristic seven conserved cysteine residues involved in the formation of the cysteine knot and the single interchain disulfide bond. Cellular responses to BMP-2 have been shown to be mediated by the formation of hetero-oligomeric complexes of type I and type II serine/threonine kinase receptors. One BMP type II and two BMP type I receptors have been identified. In contrast to the TGF-β type I receptor, which does not bind the ligand in the absence of the TGF-β receptor type II, both BMP receptor type Is can bind BMP-2 with high-affinity in the absence of BMP receptor type II. For recent reviews, see Kawabata, M. et al., 1998, Cytokine and Growth Factor Reviews 9:49; Ebendal, T. et al., 1998, J. Neurosci. Res. 51:139; and Reddi, A.H., 1998, Nature Biotechnology 16:247.