SB-431542 is a potent and selective inhibitor of ALK5 with IC50 of 94 nM in a cell-free assay, 100-fold more selective for ALK5 than p38 MAPK and other kinases. SB 431542 blocks the TGF-β-mediated growth factors, including PDGF-A, FGF-2 and HB-EGF, leading to an increase in proliferation of MG63 cells.