In neonates, variants of endothelin-1 (EDN1), carbamoyl phosphate synthetase I (CPS1), neurogenic locus notch homolog protein 3 (NOTCH3), and SMAD family member 9 (SMAD9) are associated with the risk of developing persistent PH of the newborn (PPHN) [82,83]. This evidence concerns the gene SMAD9 and persistent fetal circulation syndrome.