Even though HPA is often caused by genetic defects in the PAH gene, a small percentage of cases originate from defects in genes involved in the proper folding of PAH (such as DnaJ heat shock protein family (Hsp40) member C12, DNAJC12) or in the biosynthesis and regeneration of BH4 (such as GTP cyclohydrolase I, GCH1; 6-pyruvoyl-tetrahydropterin synthase, PTS; pterin-4 alpha-carbinolamine dehydratase 1, PCBD1; quinoid dihydropteridine reductase, QDPR) [3,7]. The gene discussed is QDPR; the disease is pulmonary arterial hypertension.