TGFB1 and idiopathic pulmonary arterial hypertension: Earlier work by other groups further strengthens this hypothesis of mixed-TGFβ/BMP receptor complexes and subsequent activation of pSMAD1/5/8 upon stimulation with TGFβ or Activins.94–97 This can be a very relevant mechanism in PAH, as not only TGFβ1, but also Activin A levels are increased in serum of IPAH and HPAH patients.89,90