SB-431542 is a selective and potent inhibitor of activin receptor-like kinase (ALK, the TGF-β type I receptor), specifically ALK4, ALK5 (IC50 = 94 nM) and ALK7. It has no effect on the other, more divergent ALK family members that recognize bone morphogenetic proteins (BMPs) such as ALK2, ALK3 or ALK6. It has also no effect on components of the ERK, JNK, or p38 MAP kinase pathways or on components of the signaling. SB431542 specifically blocks Smad signaling and suppresses renewal in embryonic and induced pluripotent stem (iPS) cells and promotes differentiation. SB431542 could enhance reprogramming efficiency when using it with MEK inhibitor PD0325901.