SMAD4 and renal fibrosis: In the canonic TGF-β pathway, once activated, TGF-β1 binds to its receptor II to activate the receptor I and the downstream regulatory Smad proteins (Smad2 and Smad3), facilitated by Co-Smad (Smad4), to translocate into the nucleus to initiate the target gene transcription and cause renal fibrosis, which is negatively regulated by Smad7 19,20.