The discovery analysis, containing 251 IPAH patients, identified rare heterozygous mutations in BMPR2 (49 cases), ACVRL1 (15 cases), TBX4 (10 cases), SMAD1 (two cases), BMPR1B (one case), KCNK3 (one case) and SMAD9 (one case). The gene discussed is SMAD9; the disease is idiopathic pulmonary arterial hypertension.