More than 350 mutations in the type-2 BMP (bone morphogenetic protein) receptor, , have been identified in patients with heritable pulmonary arterial hypertension (HPAH). However, only 30% of mutation carriers develop PAH, and we cannot predict which of these carriers will develop clinical disease. One possibility is that the nature of the mutation affects disease severity. This hypothesis has been difficult to test clinically, given the rarity of HPAH and the complexity of the confounding genetic and environmental risk factors. To test this hypothesis, therefore, we evaluated the susceptibility to experimental pulmonary hypertension (PH) of mice carrying different HPAH-associated mutations on otherwise identical genetic backgrounds. Mice with ΔEx4-5 mutations (), in which the mutant protein is not expressed, develop less severe PH in response to hypoxia or hypoxia with vascular endothelial growth factor receptor inhibition than mice with an extracellular-domain ΔEx2 mutation (), in which the mutant protein is expressed. This was associated with a marked decrease in stabilizing phosphorylation of threonine 495 endothelial nitric oxide synthase (pThr495 eNOS) in compared to wild-type and mouse lungs. These findings provide the first experimental evidence that mutation types influence the severity of HPAH and suggest that patients with mutations who express mutant BMPR2 proteins by escaping non-sense-mediated messenger RNA decay (NMD- mutations) will develop more severe disease than HPAH patients with NMD+ mutations who do not express BMPR2 mutant proteins. Since decreased levels of pThr495 eNOS are associated with increased eNOS uncoupling, our data also suggest that this effect may result from defects in eNOS function.