SMAD3 Inhibtor, SIS 3-d3 is the labeled analogue of SMAD3 Inhibtor, SIS 3 (S564500), which selectively inhibits TGF-β and activin. It has been shown to reduce TGF-β1-induced type 1 procollagen expression and myofibroblast differentiation in normal dermal fibroblasts as well as scleroderma fibroblasts.