BACKGROUND: Pulmonary hypertension (PH) is a progressive cardiopulmonary disorder characterized by vascular remodeling, abnormal vasoconstriction of small lung arteries, and right heart failure. Hypoxia causes vascular damage, leading to vessel stenosis or occlusion by aberrant endothelial cells, hypertrophy of the tunica media, and thrombus formation. But the precise molecular mechanisms underlying the pathology of PH have been uncertain. METHODS: To investigate the pathogenic role of Myl (myosin light chain) 9/12 in PH, we utilized the Sugen/hypoxia mouse model, generated by administration of the VEGF (vascular endothelial growth factor) receptor inhibitor SU5416 under hypoxic conditions (10% O2). Lung tissues of patients with PH and human lung microvascular endothelial cells were used to examine their endothelial changes.