Selective inhibitor of the TGF-β type I receptor ALK5 (IC50 values are 0.004 and 0.023 μM for ALK5 autophosphorylation and ALK5 binding respectively). Selective for ALK5 over a range of kinases, including p38 MAPK, JNK1 and GSK3 (IC50 > 16 μM). Enhances the efficiency of cellular reprogramming.