Phenylalanine hydroxylase deficiency is an inherited disorder that is characterized by increased levels of phenylalanine in the blood. This is due to a lack of the enzyme phenylalanine hydroxylase. If untreated, individuals with phenylalanine hydroxylase deficiency can develop symptoms including intellectual disability, seizures, behavioral problems, and psychiatric disorders. Untreated individuals may have a musty or mouse-like odor as a side effect of excess phenylalanine in the body. Women who have phenylalanine hydroxylase deficiency and are pregnant must be carefully managed to reduce risk of exposure of high levels of phenylalanine to the baby. The severity of this disease can be decreased significantly by a low-protein diet and use of a Phe-free medical formula as soon as possible after birth. If identified right after birth and diet treatment is strictly followed, the outcome of children with phenylalanine hydroxylase deficiency is expected to be good. Delayed treatment can damage the brain and cause irreversible intellectual disability and psychiatric problems. Phenylalanine hydroxylase deficiency is caused by pathogenic variants in the PAH gene.