PKU is mostly due to pathogenic variants of the phenylalanine hydroxylase (PAH) gene but hyperphenylalaninemia (HPA) may also be caused by mutations in tetrahydrobiopterin (BH4) metabolism [2] or in the DNAJC12 gene which encodes a co-chaperonne interacting with the PAH [3]. The gene discussed is PAH; the disease is pulmonary arterial hypertension.