This mouse mutation, which causes hyperphenylalaninemia, does not affect the gene encoding PAH. PAH activity is only slightly reduced, not sufficiently to cause a PKU phenotype (J:9146). Mutant animals respond to the administration of BH4, and their livers are deficient in this coenzyme. DHPR is present in normal quantities in these animals, but GCH activity is practically absent in young hph1/hph1 homozygotes, and greatly reduced in older animals. Heterozygotes are intermediate in activity between homozygous normal and hph1 homozygotes. Thus, GCH deficiency seems to be the underlying cause of HPH in hph1 mutant mice (J:8982). BH4 is also a cofactor for tryptophane hydroxylase and tyrosine hydroxylase, which are rate limiting enzymes in the synthesis of serotonin and dopamine. The hph1 mutation has been associated with reduced levels of these substances and their metabolites in liver and brain. It has been suggested that neuropathological mechanisms in dopa-responsive dystonia are related to BH4 activity, and are amenable to study in hph1 mice (J:34167).