An ALK1 and ALK2 inhibitor (IC50s = 1.8 and 1.1 nM, respectively) that can prevent BMP4-induced SMAD1/5/8 pathway activation in vitro (IC50 = 100 nM) without affecting TGF-β signaling; less selective for ALK3, 4, 5, and 6 subtypes and the type II BMP receptor ActRIIA (IC50s = 34.4, 302, 321, 6.4, and 220 nM, respectively).