Anti-TGF-β antibody significantly reduced Ser423/425 Smad3 phosphorylation rates in non-stimulated PAH PAVSMC and in PDGF-BB stimulated non-diseased cells but had little effect on Smad2 and Smad1/5 phosphorylation, seen in Figure 4. Here, SMAD1 is linked to pulmonary arterial hypertension.