Potent and selective inhibitor of the transforming growth factor-β (TGF-β) type I receptor activin receptor-like kinase ALK5 (IC50 = 94 nM), and its relatives ALK4 and ALK7. Suppresses TGF-β-induced proliferation of human osteosarcoma cells. Stimulates proliferation, differentiation and sheet formation of ESC-derived endothelial cells.