A selective ALK2 inhibitor (IC50 = 1.3 nM) that less potently inhibits ALK1 and ALK3 (IC50 = 2.4 and 85.8 nM, respectively) and demonstrates over 1,500-fold selectivity against the closely related activin and TGF-β type I receptors (i.e., ALK4 and ALK5); inhibits BMP6-induced osteogenic differentiation with an IC50 value of 10 nM.