Over 1500 PAH variants are currently known (PAHvdb: Phenylalanine Hydroxylase Gene Locus-Specific Database, http://www.biopku.org/home/pah.asp; accessed on 3 March 2023), and depending on the inherited alleles, affected individuals may have a very mild to a pronounced increase in Phe [27]. Here, PAH is linked to pulmonary arterial hypertension.