SMAD3 and pulmonary arterial hypertension: Together with our data showing that both Smad2 and Smad3 phosphorylation is already diminished in hyper-proliferative human PAH PAVSMC, shown in Figure 2, these findings failed to explain growth-inhibitory effects of anti-Activin A and anti-TGF-β antibodies in human PAH PAVSMC, as shown in Figure 3A.