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. Here, ACVRL1 is linked to heritable pulmonary arterial hypertension.