Phenylketonuria (PKU) is an inborn error of metabolism that results in decreased metabolism of the amino acid phenylalanine. Untreated, PKU can lead to intellectual disability, seizures, behavioral problems, and mental disorders. It may also result in a musty smell and lighter skin. Babies born to mothers who have poorly treated PKU may have heart problems, a small head, and low birth weight.Phenylketonuria is a genetic disorder inherited from a person's parents. It is due to mutations in the PAH gene, which results in low levels of the enzyme phenylalanine hydroxylase. This results in the buildup of dietary phenylalanine to potentially toxic levels. It is autosomal recessive, meaning that both copies of the gene must be mutated for the condition to develop. There are two main types, classic PKU and variant PKU, depending on if any enzyme function remains. Those with one copy of a mutated gene typically do not have symptoms. Many countries have newborn screening programs for the disease.Treatment is with a diet low in foods that contain phenylalanine and special supplements. Babies should use a special formula. The diet should begin as soon as possible after birth and be continued for at least 10 years, if not lifelong. People who are diagnosed early and maintain a strict diet can have normal health and a normal life span. Effectiveness is monitored through periodic blood tests. The medication sapropterin dihydrochloride may be useful in some.Phenylketonuria affects about 1 in 12,000 babies. Males and females are affected equally. The disease was discovered in 1934 by Ivar Asbjørn Følling, with the importance of diet determined in 1953. Gene therapy, while promising, requires a great deal more study as of 2014.