TH and phenylketonuria: Manipulation of downstream regulatory enzymes of L-Phe conversion (e.g., tyrosine hydroxylase), involved in specific neurotransmitter synthesis (like dopamine) and that have reduced expression levels in the brains of Pahenu2 PKU mouse models (Joseph and Dyer, 2003; Emburya et al., 2007), could also allow a better understanding on the neurotransmitter dysfunction reported in PKU.