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From SNPedia

Hyperphenylalaninemia is broadly defined as the presence of blood phenylalanine levels that are greater than the normal limits of the upper reference range (2 mg/dL or 120 mmol/L) yet are lower than the levels seen in patients with classic phenylketonuria (PKU), which are higher than 20 mg/dL (1200 mmol/L). [1]

Both hyperphenylalaninemia and phenylketonuria are caused by mutations in the phenylalanine hydroxylase PAH gene on chromosome 12, and both are considered to be inherited in an autosomal recessive manner. Up to 10% of newborn children with a positive Guthrie test have hyperphenylalaninemia, which according to most sources (but not all) does not require dietary treatment. Depending on the relative frequencies of different PAH mutations in a particular population, hyperphenylalaninemia is usually caused by the combined effect of a mild hyperphenylalaninemia mutation and a severe PKU mutation. Phenylketonuria is usually caused by the combined effect of having inherited two PKU mutations, usually in a compound heterozygous form.