To the Editor: Pulmonary arterial hypertension (PAH) is a disease in which remodelling of the small pulmonary arteries leads to an increase in pulmonary artery pressure (PAP). The most important genetic predisposing factor related to PAH is a mutation in the bone morphogenetic protein receptor type 2 gene ( BMPR2 ) [1, 2]. BMPR2 mutation carriers are known to present with disease at an earlier age and with worse haemodynamics [3]. We recently showed in a cohort of patients with idiopathic and hereditary PAH that a very low diffusion capacity for carbon monoxide ( D LCO) is exclusively found in some of the patients without identified BMPR2 mutations, whereas BMPR2 mutation carriers have a relatively preserved D LCO [4]. D LCO is a noninvasive marker of the quality of the alveolar capillary structure [5] and the observed difference in D LCO supports the hypothesis that distinct vascular disease processes are at play in BMPR2 mutation-related PAH and non BMPR2 mutation-related idiopathic PAH. Until recently, insufficient availability of lung samples has prohibited the performance of a detailed comparison of the pulmonary vascular pathologies in these two disease groups [6]. Therefore, we sought, in the present study, to confirm the previously found influence of BMPR2 mutations on diffusion capacity in a much larger multinational patient cohort. We performed a retrospective collaborative study at the VU University Medical Center in Amsterdam, the Netherlands and the Universite Paris-Sud, Assistance Publique Hopitaux de Paris, Le Kremlin-Bicetre, France. Patients were eligible for this study when classified in the database with idiopathic or familial PAH, and when the results from BMPR2 mutation analysis and D LCO measurements were available. Patients were diagnosed with idiopathic …