In more severe cases, clonal endothelial lesions, known as “plexiforms,” which consist of highly proliferative endothelial cells can also develop in the lungs.2, –4 A deregulation of vasoactive mediators released by endothelial cells in pulmonary arteries underpins PAH, with elevated levels of the constrictor peptide, endothelin (ET)-1 being a key therapeutic target in the treatment of the disease. Here, EDN1 is linked to pulmonary arterial hypertension.