SMAD3 and renal fibrosis: Briefly, let-7b expression is reduced in the mouse models of renal fibrosis that upregulate TGF-β1 receptor 1 (TGFBR1), let-7b directly represses TGFBR1, and extracellular matrix proteins expression, it also decreases SMAD3 activity, and attenuates the profibrotic effects of TGF-β1 (21).