PKU is caused by a defect in the gene that helps create the enzyme needed to break down phenylalanine. Without the enzyme necessary to process phenylalanine, a dangerous buildup can develop when a person with PKU eats foods that contain protein or eats aspartame, an artificial sweetener.

What enzyme causes phenylketonuria?

Mutations in the PAH gene cause phenylketonuria. The PAH gene provides instructions for making an enzyme called phenylalanine hydroxylase . This enzyme converts the amino acid phenylalanine to other important compounds in the body.

When does phenylketonuria develop?

Different forms of phenylketonuria vary in their severity of signs. Classic phenylketonuria (PKU) is the most severe form. Babies with PKU usually seem healthy at birth. Signs of PKU begin to appear around six months of age.