Bone morphogenetic proteins (BMPs) were first identified as molecules that can induce ectopic bone and cartilage formation. BMPs belongs to the TGF-β superfamily, playing many diverse functions during development. BMPs are synthesized as precursor proteins and then processed by cleavage to release the c-terminal mature BMP. BMPs initiate signaling by binding to a receptor complex containing type I and type II serine/threonine receptor kinases that then phosphorylate Smad (mainly Smad1, 5 and 8), resulting the translocation of Smad into the nucleus. BMP was also reported to activate MAPK pathways in some systems.