L-Biopterin is an important cofactor in the production of many essential neurotransmitters, such as dopamine, seratonin and epinephrine, and plays an important role in the production of nitric oxide in the body. It is produced as an oxidation product by the melanogenesis regulating cofactor (6R)5,6,7,8 tetrahydrobiopterin (6-BH4). L-Biopterin has been shown to be cytotoxic to melanocytes in vitro, and may be a useful noninvasive biomarker.