TGFB1 and pulmonary arterial hypertension: Dysregulation of the transforming growth factor-β (TGF-β) superfamily—particularly an imbalance between anti-proliferative BMP/BMPR2–Smad1/5/8 and pro-proliferative activin/ActRIIA–Smad2/3 signalling—is central to PAH pathobiology [7,8,9,10].