BH4-deficient hyperphenylalaninemia A is an inherited disorder caused by a defective enzyme called 6-pyruvoyltetrahydropterin synthase that results in increased levels of a protein building block called phenylalanine. As a result, it alters levels of molecules called neurotransmitters, which are necessary for communication between nerve cells in the brain. Affected individuals appear normal at birth or may be smaller than normal at birth. Symptoms begin to appear within a few days to a few months after birth. Symptoms can range from mild to severe and become more evident over time. They include low muscle tone, progressive problems with development, difficulty swallowing, seizures, movement disorders, behavioral problems, an inability to control body temperature, and intellectual disability. Initiation of treatment of dietary restrictions, along with supplementation of BH4 and other compounds is able to prevent motor and intellectual decline and a normal life expectancy is expected. Without treatment, early death is possible. BH4-deficient hyperphenylalaninemia is caused by pathogenic variants in the PTS gene.