The main purpose is to test whether treatment with BH4-tablets can replace the protein restrictive diet in patients with mild PKU caused by a certain frequent mutation.

PKU, phenylketonuria, is a rare, inherited metabolic disorder that results in mental retardation if not a very strict low-protein diet is started within the first weeks of life.

The conversion of phenylalanine, phe, to tyrosine is defect, phe accumulates, leading to brain damage. There are different degrees of severity, reflecting the spectrum of mutant genes. BH4, tetrahydrobiopterin, is a co-enzym for the conversion of phe to tyrosine. It is known that BH4 can lower phe in some patients with milder forms of PKU.

The main purpose is to test whether treatment with oral BH4 can replace the protein restrictive diet in patients with mild PKU caused by the frequent mutation Y414C in the phenylalanine hydroxylase gene.

Inclusion Criteria:

Diagnosed mild PKU with 2 known mutations in the PAH gene, hereof at least one Y414C From the age of eight Informed consent -

Exclusion Criteria:

Less than 8 years of age Pregnancy or treated with the intensive diet of pregnancy

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