The TGF-β1–Smad2/3 signaling can lead to renal fibrosis by the activation of Snail, and the δ-opioid receptor can act like an antifibrotic factor regulating the Snail gene from TGF-b/Smad, p38, and Akt signaling pathways (61). The gene discussed is SMAD2; the disease is renal fibrosis.