Similar frequencies of rare, deleterious BMPR2 mutations were observed in paediatric- and adult-onset I/FPAH patients; however, there was significant enrichment of rare, predicted deleterious TBX4 mutations in paediatric- (10 of 130 patients) compared with adult-onset (0 of 178 patients) IPAH patients. The gene discussed is BMPR2; the disease is idiopathic pulmonary arterial hypertension.