The penetrance of a genetic disorder is the risk for disease onset, given the disease-causing mutation. Heritable pulmonary arterial hypertension (PAH) is a rare genetic disorder characterised by mutations mostly occurring in the bone morphogenetic protein receptor type 2 (BMPR2) gene, and a wide heterogeneity of penetrance modifier mechanisms. In this study, we genotyped 32 individuals, including 12 healthy BMPR2 mutation carriers and 5 patients, from a large Iberian family affected by PAH. Using genetic linkage analysis techniques and functional genomics data, we identified a candidate genetic modifier of heritable PAH. This result may help to advance our understanding of genetic modifiers of penetrance in heritable PAH. (By Dr. Robert Castelo, https://jmg.bmj.com/content/early/2019/03/20/jmedgenet-2018-105669 )