1,2,3,4-Tetrahydro-6,7-dimethoxy-2-[(2E)-3-(1-methyl-2-phenyl-1H-pyrrolo[2,3-b]pyridin-3-yl)-1-oxo-2-propenyl]-isoquinoline Monohydrochloride is a potent and selective Smad3 inhibitor that attenuates transforming growth factor (TGF)-β1-induced phosphorylation of Smad3 and interaction of Smad3 with Smad4 (1). It also inhibits TGF-β1-induced myofibroblast differentiation of dermal fibroblasts and TGF-β2-induced endothelial cell differentiation in iPSCs (1,2).