Bone morphogenetic proteins is a pleiotropic cytokine belonging to the superfamily of transforming growth factor type B. They are bound by two different types of receptors with an activity of serine-threonine kinase. Intracellular second messenger signal are Smad proteins. R-Smad phosphorylation leads to the formation of a complex with Co-Smad, which is transported to the nucleus and regulates transcription of selected genes. The transmission signal is modulated by a variety of biological factors.