MCCC2 and pulmonary arterial hypertension: Considering established recessive SysNDD genes, top-ranked genes (frequency > 1%) included PAH, KIAA0586, PMM2, DHCR7, and MCCC2 (Supplementary Table 1a), most of which exert biochemical phenotypes, with PAH, PMM2 and DHCR7 being treatable.