SMAD9 and pulmonary arterial hypertension: Rare deleterious variants in SMAD9 lead to approximately 0.9%–2.1% childhood-onset PAH and predominantly occur in IPAH cases (Levy et al., 2016; Zhu et al., 2018a; Zhu et al., 2018b; Graf et al., 2018; Zhu et al., 2019; Abman et al., 2022).