Phenylketonuria (PKU) is the most common genetic enzyme defect, with an overall incidence in Europe and the USA of 1:10,000-20,000 live births per year. Patients suffer from a genetic defect in the liver enzyme phenylalanine hydroxylase (PAH), which normally metabolizes the amino acid phenylalanine (Phe) into the amino acid tyrosine. This specific enzyme defect, which results in an increase in the ...