The last 30 years of pulmonary hypertension research is a qualified translational success story. Patients with portopulmonary hypertension (PoPH) have benefitted from inclusion in the licensed indications for novel therapies, despite small numbers of patients enrolled in studies (1). There has however, been very little movement in our parallel understanding of the pathophysiology. Perhaps as a consequence of this gap between evolving treatments primarily aimed at other disease causes, and our lack of mechanistic insight in PoPH, outcomes for patients remain poor. Modern registry data demonstrates 5-year survival stuck around 40% for patients with PoPH, in contrast to the improving survival in other disease forms (2). With a paucity of funded research, limited preclinical modelling and no real external drivers for industry to engage in this question rather than focus resources on subsets of patients in phase 3 trials, there have been little in the way of new hypotheses to consider. An added complication is that patients have 2 disease processes, pulmonary hypertension and liver disease, and the relationship between the degree and nature of liver disease, splanchnic and pulmonary pressures has not been clearly resolved (3). In this edition of the journal, Nikolic et al report a potentially fundamental advance in our understanding of disease (4). BMP9, a ligand of the TGF superfamily that has a selective binding affinity to the BMPR2/ALK1 complex, is significantly reduced in PoPH but not in other forms of PAH. BMP9 is emerging as an important and novel regulator of vascular homeostasis (5). The concept that BMP signalling may be important in the liver vasculature has clear precedent. Hereditary haemorrhagic telangiectasia (HHT) is characterised by arteriovenous malformations that affect organs heterogeneously. They are found commonly in the liver and HHT is associated in around 80% with mutations in ALK1 and the circulating co-receptor Endoglin (6). In addition to the established link between HHT, BMP signalling and PAH, the genetics of PAH have been pointing for some time to the critical importance of this specific ligand and its receptor complex. Completing the tertiary receptor/ligand complex, mutations in BMPR2 and BMP9 cause PAH (7). Fitting beautifully with the human genetics, the BMPR2/ALK1/Endoglin tertiary complex is highly expressed in the pulmonary endothelium, and BMP9 circulates at physiological levels (8). To complete the background story, BMP9 is produced predominantly by the liver (9).