TWIST1 and idiopathic pulmonary arterial hypertension: Twist1 knockdown in IPAH patient PAECs suppressed PDGFB expression and accumulation of αSMA-positive cells in the gel (Fig. 5b), suggesting that accumulation of αSMA-positive cells in the gel is inhibited in IPAH-patient derived PAECs by suppressing Twist 1-PDGFB signaling.