Furthermore, in plexiform lesions from PAH patients, expression of CD39/ENTPD1, the ectonucleotidase responsible for the conversion of the nucleotides ATP and ADP to AMP, was decreased, suggesting that increased extracellular ATP level in the pulmonary vascular endothelium and within the angiomatoid proliferative lesions might contribute to excessive endothelial proliferation [44]. Here, ENTPD1 is linked to pulmonary arterial hypertension.