BMP-2 is one of at least 20 structurally and functionally related BMPs, which are members of the transforming growth factor beta (TGF-beta ) superfamily (1, 2). BMPs were originally indentified as protein regulators of cartilage and bone formation. However, they are also involved in embryogenesis and morphogenesis of various tissues and organs. BMPs 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-beta 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, while mature human BMP-2 and zebrafish BMP-2a are 85% identical. Zebrafish have another homolog of BMP-2a, BMP-2b, which is 88% identical to BMP-2a in the mature region and corresponds to the swirl mutant. The combined expression pattern of zBMP-2a/2b/4 coincides with areas where BMP-2/4 expression would be found in other vertebrates (3). Biologically active BMP-2a is a disulfide-linked homodimer of the carboxy-terminal 115 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-2a have been shown to be mediated by the formation of hetero-ogliomeric complexes of type I and type II serine/threonine kinase receptors (4). In contrast to the TGF-beta type I receptor, which does not bind the ligand in the absence of the TGF-beta receptor type II, both BMP receptor type I’s can bind BMP-2 with high affinity in the absence of BMP receptor type II.