This disorder occurs through a mutation in the SPR gene, which is responsible for encoding the sepiapterin reductase enzyme. The enzyme is involved in the last step of producing tetrahydrobiopterin, better known as BH. BH is involved in the processing of amino acids and the production of neurotransmitters, specifically that of dopamine and serotonin which are primarily used in transmission of signals between nerve cells in the brain. The mutation in the SPR gene interferes with the production of the enzyme by producing enzymes with little or no function at all. This interference results in a lack of BH specifically in the brain. The lack of BH only occurs in the brain because other parts of the body adapt and utilize alternate pathways for the production of BH. The mutation in the SPR gene leads to nonfunctional sepiapterin reductase enzymes, which results in a lack of BH and ultimately disrupts the production of dopamine and serotonin in the brain. The disruption of dopamine and serotonin production leads to the visible symptoms present in patients suffering from sepiapterin reductase deficiency. SR deficiency is considered an inherited autosomal recessive condition disorder because each parent carries one copy of the mutated gene, but typically do not show any signs or symptoms of the condition.