Since GTPCH1 is the rate-limiting enzyme in BH4 biosynthesis, the expression of the gene and the protein amount was evaluated in human cell lines corresponding to distinct stages of melanoma progression: melanocytes, WM1552C (RGP), WM793, and WM1366 (VGP), WM983B, and Lu1205 (MET). The gene discussed is GCH1; the disease is melanoma.