SB 431542 is an inhibitor of the transforming growth factor-beta 1 (TGF-β1) activin receptor-like kinases (ALKs). It is a selective and potent inhibitor of ALK-4, -5 (IC50 = 94 nM) and -7. SB431542 inhibits endogenous activin and TGF-β signaling without affecting more divergent BMP signaling utilizing ALK-1, -2, -3, and -6 (1,2). 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. SB 431542 at 1μM stimulates proliferation, differentiation, and sheet formation of endothelial cells derived from embryonic stem cells (3).