PAH deficiency, although a rare inborn disorder of metabolism, is among the most common disorders detected through newborn screening, with a global prevalence of approximately 1:24?000 livebirths1 and an estimated half a million individuals affected worldwide. PAH deficiency is also colloquially known as phenylketonuria because of its association with urinary phenylpyruvate excretion, a detectable phenotype that was used clinically for diagnosis before the advent of amino acid analysis and measurement of blood L-Phe.