Cytokines of the TGF-β family exert multiple biological effects by their signal transduction pathways that undergo a basic course as follows: ligands of TGF-β family→receptors→SMADs→transcription factors→DNA expression. In stimulating their own two type receptors with kinase activity the cytokines first bind to type-Ⅱ receptors and then the ligand-binding type-Ⅱ receptors activate type-Ⅰ receptors. The type-Ⅰ receptors phosphorylate pathway specific SMADs. These activated SMADs then associate with common SMAD and translocate from cytoplasm to nucleus where they regulate transcription either by associating with transcription factors and by binding directly to DNA.