NOTCH3 and idiopathic pulmonary arterial hypertension: NOTCH3-ECD levels were significantly higher in left atrial serum compared to serum from the left or right main pulmonary artery in patients with IPAH, suggesting that the pulmonary vasculature is the source of NOTCH3-ECD (Extended Data Fig. 5), where NOTCH3 is known to be highly expressed7.