Primary pulmonary hypertension (PPH), characterized by obstruction of pre-capillary pulmonary arteries, leads to sustained elevation of pulmonary arterial pressure (mean >25 mm Hg at rest or >30 mm Hg during exercise). The aetiology is unknown, but the histological features reveal proliferation of endothelial and smooth muscle cells with vascular remodelling (Fig. 1). More than one affected relative has been identified in at least 6% of cases (familial PPH, MIM 178600). Familial PPH (FPPH) segregates as an autosomal dominant disorder with reduced penetrance and has been mapped to a locus designated PPH1 on 2q33, with no evidence of heterogeneity. We now show that FPPH is caused by mutations in BMPR2, encoding a TGF-β type II receptor (BMPR-II). Members of the TGF-β superfamily transduce signals by binding to heteromeric complexes of type I and II receptors, which activates serine/threonine kinases, leading to transcriptional regulation by phosphorylated Smads. By comparison with in vitro studies, identified defects of BMPR-II in FPPH are predicted to disrupt ligand binding, kinase activity and heteromeric dimer formation. Our data demonstrate the molecular basis of FPPH and underscore the importance in vivo of the TGF-β signalling pathway in the maintenance of blood vessel integrity.†These authors contributed equally to this work. *Micheala Aldred2, Christopher A. Brannon3, P. Michael Conneally4, Tatiana Foroud4, Neale Fretwell2, Radhika Gaddipati1, Daniel Koller4, Emily J. Loyd1, Neil Morgan2, John H. Newman1, Melissa A. Prince1, Carles Vilariño Güell2 & Lisa Wheeler1 1Vanderbilt University Medical Center, Nashville, Tennessee, USA. 2Division of Medical Genetics, Departments of Genetics and Medicine, University of Leicester, UK. 3Division of Human Genetics, Children's Hospital Medical Center, Cincinnati, Ohio, USA. 4Indiana University School of Medicine, Indianapolis, Indiana, USA. Correspondence should be addressed to J.E.L. (e-mail: Jim.Loyd@mcmail.vanderbilt.edu), W.C.N. (e-mail: bill.nichols@chmcc.org) or R.C.T. (e-mail: rtrembat@hgmp.mrc.ac.uk).