SMAD7 and Hepatic fibrosis: Additionally, Dooley et al. illustrated a relationship between miR-21 and hepatic fibrosis through transforming growth factor β 1 (TGF-β1), a critical mediator of hepatic fibrogenesis, and it stimulates the expression of miR-21, and that miR-21 reduces the expression of SMAD7, a negative regulator of TGF-β signaling [28].