Only in 3 out of the 22 families with HPAH (13.6%, Figure 3) examination of the BMPR2 gene (promoter and coding regions including flanking intronic regions) and the coding regions of the ACVRL1, ENDOGLIN, and the SMAD8 genes did not reveal any defect. The gene discussed is SMAD9; the disease is heritable pulmonary arterial hypertension.