PAH and phenylketonuria: The Pah-R261Q knock-in mouse that carries a frequent mutation in HPA/PKU patients exhibits reduced total hepatic PAH activity and presents phenotypic traits characteristic of homozygous patients with the R261Q:R261Q genotype, such as increased L-Phe and decreased L-Trp and L-Tyr in blood compared to WT, sensitivity to L-Phe challenge, and responsiveness to BH4 supplementation (http://www.biopku.org)8,16.