PAH and pulmonary arterial hypertension: Phenylketonuria (PKU; OMIM 261600) is a recessive genetic disease of amino acid (AA) metabolism caused by loss of function mutations of the gene encoding phenylalanine hydroxylase (EC 1.14.16.1, PAH in humans and Pah in mice), resulting in an inability to convert phenylalanine (phe) to tyrosine [1].