Bone morphogenetic proteins (BMPs) are osteotro-phic and osteoinductive cytokines. BMPs are pleio-tropic cytokines with actions not only on bone and cartilage but also on brain, teeth, skin, heart, lung, kidney, and a host of other tissues. They act on chemotaxis. mitosis, differentiation, cell survival and cell death. The biological actions of BMPs are med-iated by binding to specific BMP receptors types I and II. There is a collaboration between the receptors and downstream substrates such as receptor-regulated R-Smads 1, 5, and 8. The phosphorylated R-Smads partner with Co-Smad. Smad4. and enter the nucleus to initiate the transcription of BMP-responsive genes. Smad6 inhibits the activity of type I BMP receptor protein kinase. Thus, the BMP receptors provide a finely regulated homeostatic system with checks and balances.