Unlike female endothelial cells, where higher XIST levels correlated to more proliferative endothelial phenotype, in males PAH patients, endothelial cells with low XIST were more proliferative than the endothelial cells with high XIST, suggesting that in male patients, augmented XIST expression may provide protection against endothelial hyperproliferation, consistent with a sex-specific relationship between XIST abnormal expression and diseases. Here, XIST is linked to pulmonary arterial hypertension.