MKI67 and pulmonary arterial hypertension: To further understand the role of endothelial hyperproliferation in the pathogenesis of PAH, we confirmed that pulmonary vascular endothelial cells (PVECs) were pro-proliferative as evidenced by increased Ki67+/CD31+ cells in the pulmonary vascular bed in Egln1Tie2Cre mice (Figure 1A).