The design of an inhibitor for the enzyme thymidylate synthase is a representation for how de novo drug design and structure-based drug design can go hand-in-hand. In the human body, this enzyme uses the coenzyme, 5,10-methylentetrahydrofolate, to catalyze the addition of a methyl group to the substrate deoxyuridylate monophosphate (dUMP). The product of this reaction is deoxythymidylate monophosphate (dTMP). Because molecules that inhibit thymidylate synthase tend to display anti-tumor properties,