Phenylalanine hydroxylase (PAH; E.C.1.14.16.1) deficiency is an autosomal recessive inherited defect that results in abnormally high blood levels of the essential amino acid phenylalanine (Phe) and causes a spectrum of hyperphenylalaninemia (HPA), whose most severe form is phenylketonuria (PKU, phenotype MIM number #261600). Here, PAH is linked to phenylketonuria.