Pteridine synthesis pathway and gene duplications in vertebrates. Pteridine synthesis contains three component pathways : the de novo synthesis of H4biopterin from GTP (top line), the H4biopterin regeneration pathway (grey) and the synthesis of yellow pteridine pigments. The formation of orange drosopterin has not been elucidated yet in vertebrates. In Drosophila, the clot enzyme is involved , which corresponds to the vertebrate Txnl5 protein. Asterisks indicate hypothetical reactions and question marks unidentified enzymes. Red indicates duplications during the fish-specific genome duplication, blue other types of duplication.