Classical PKU is caused by the deficiency of phenylalanine hydroxylase (PAH, EC 1.14.16.1) resulting in accumulation of excess phenylalanine (Phe) from dietary protein and causing the condition of hyperphenylalaninaemia (HPA), which is the main cause of neurological disturbances and intellectual disability in cases of untreated PKU [1]. The gene discussed is PAH; the disease is phenylketonuria.