The PAH gene produces an enzyme that breaks down the amino acid phenylalanine. Mutations in the PAH gene can cause phenylketonuria (PKU), a disorder that can change cells in the brain. The faulty protein allows dangerously high levels of phenylalanine to accumulate in the brain, poisoning the cells. If a person with PKU consumes too much phenylalanine, the build-up can cause mental retardation.

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