Sepiapterin reductase (SPR) is a key enzyme in the brain's BH4 synthesis pathway, belonging to the short-chain dehydrogenase/reductase family. SPR primarily consists of a single structural domain and is mainly expressed in the nervous system and liver. This enzyme is involved in the reduction of sepiapterin to tetrahydrobiopterin (BH4), an essential cofactor for several amino acid hydroxylases. The expression of the SPR gene is regulated by cell type and developmental stage, and mutations can lead to disorders in neurotransmitter synthesis, such as dopamine.