Inhibits BMP type 1 receptor-induced phosphorylation of SMAD1/5/8 (IC50 = 4.9 nM); shows specificity for ALK1, 2, 3, and 6 (IC50s = 0.8, 0.8, 5.3, and 16.7 nM, respectively) over ALK4 and 5 (IC50s = 101 and 350 nM, respectively).