SMAD9 and idiopathic pulmonary arterial hypertension: The innovation of gene detection technology has helped to identify novel genes associated with childhood-onset IPAH, including T-box 4 (TBX4), SRY-related HMG box transcription factor (SOX17), mothers against decapentaplegic 9 (SMAD9), potassium two-pore-domain channel subfamily K member 3 (KCNK3), caveolin-1 (CAV1), and growth differentiation factor 2 (GDF2) (Chida et al., 2012; Garcia-Rivas et al., 2017; Zhu et al., 2018a; Morrell et al., 2019; Rosenzweig et al., 2019; Montani et al., 2021).