GCH1 and pulmonary arterial hypertension: After Sanger sequencing, patients lacking at least one identified variant should be examined via MLPA to screen for exon deletions/duplications within PAH. Lastly, if patients presenting with high phenylalanine levels cannot be diagnosed by PAH gene analysis, next-generation sequencing of genes related to BH4 deficiency, including PTS, GCH1, PCBD1, QDPR, and GFRP, should be applied.