ACVRL1 and pulmonary arterial hypertension: Since TGF-β/ALK-1/Smad1,5,8 signaling requires the kinase activity of TβRI, the effect of TGF-β signaling on endothelial function may be regulated by the balance of both signals and, in turn, BMP-9/ALK-1 signaling could modulate TGF-β signaling in ECs, even in the pathology of PAH.