While there have been some reports that eNOS expression is diminished in the lungs of humans with PAH [121] along with reduced levels of NO in the exhaled breath of individuals with PAH [122,123], others have shown no change or increased expression of eNOS in pulmonary vessels [124,125,126,127] and this may reflect the different etiologies of PAH. Here, NOS3 is linked to pulmonary arterial hypertension.