Imbalanced signaling by the transforming growth factor-beta (TGF-β) superfamily significantly contributes to dysregulated vascular cell proliferation in PAH, characterized by overactive pro-proliferative mothers against decapentaplegic homolog 2/3 (Smad2/3) signaling occurring concurrently with deficient antiproliferative mothers against decapentaplegic homolog 1/5/8 (Smad1/5/8) signaling. Here, SMAD1 is linked to pulmonary arterial hypertension.