Interestingly, in idiopathic PAH (iPAH) mRNA and protein levels of ALK1 and endoglin are specifically increased in ECs, leading to enhanced Smad1/5 phosphorylation (pSmad1/5) when stimulated with TGF-β, indicating a disturbed TGF-β/BMP balance [19]. Here, TGFB1 is linked to idiopathic pulmonary arterial hypertension.