SMAD1 and pulmonary arterial hypertension: Additional PAH risk genes include those encoding the BMPRII ligands BMP9 and BMP10 (46–48); the BMPRII signaling partners activin receptor-like kinase 1 (ALK1) and endoglin (49–51); the BMPRII transcriptional mediators SMAD1, SMAD4, and SMAD8 (52, 53); and the scaffolding protein caveolin-1 (54–56), which regulates BMPRII signaling through its localization and internalization.