Transforming growth factor (TGF)β signaling generally occurs through TGFβ type I and type II receptors and Smads. TGFβ binds to the TGFβ type I and type II receptors. The type I receptor contains kinase activity, and phosphorylates receptor-activated Smads, Smad2 and Smad3, which dimerize with Smad4. The resultant complex migrates into the nucleus to activate target gene expression. TGFβ induces the inhibitory Smads, Smad6 and Smad7, which block TGFβ receptor type I-dependent Smad 2/3 activation.