We reported that apoptosis-resistant PAECs isolated from monocrotaline injection following left pneumonectomy-induced PAH rats or induced by hypoxia treatment demonstrated increased phosphorylation of STAT3 at Tyr 705, and when STAT3 phosphorylation inhibited by AG-490, the apoptosis increased in PAECs cultured in a 5% O2 environment. This evidence concerns the gene STAT3 and pulmonary arterial hypertension.