SMAD9 and heritable pulmonary arterial hypertension: Since 2000 in HPAH-patients and families several mutations of genes of the transforming growth factor beta (TGF-β) superfamily of receptors have been found as in the bone morphogenetic protein receptor 2 (BMPR2) gene [3]–[5], Activin A receptor type II-like 1 (ACVRL1, also called ALK1) [6], Endoglin[7], and SMAD9[8] (also called SMAD8).