In contrast to classical forms of BH4 deficiency DRD and SR deficiency present without hyperphenylalaninemia and thus cannot be detected by the neonatal screening for phenylketonuria (PKU).
Thus, autosomal recessive SR deficiency leads to BH(4) and to neurotransmitter deficiencies without hyperphenylalaninemia and may not be detected by neonatal screening for phenylketonuria.