Description; Application; 6-Biopterin is an oxidation product from the melanogenesis regulating cofactor (6R)5,6,7,8 tetrahydrobiopterin (6-BH4). In vitro, 6-Biopterin is cytotoxic to melanocytes. However, in vivo, 6-Biopterin is rapidly and sequentially reduced back to 6-GH4 via q-BH2. Recent studies suggest that 6-Biopterin among other pteridines may be a useful non-invasive biomarker for cancer.