Phenylalanine hydroxylase (PAH; EC 1.14.16.1) deficiency commonly called phenylketonuria (PKU) is an autosomal recessive inborn error of Phe metabolism resulting in an inability to synthesize Tyr and elevated concentrations of its precursor Phe in blood and brain (Singh et al. 2014; Vockey et al. 2014). The gene discussed is PAH; the disease is phenylketonuria.