Well, inside our body, the cells have to be able to synthesize purine molecules and pyrimidine molecules because these are the molecules that are basically used to produce DNA molecules. Now, one important enzyme in the biosynthesis of purines and pyramidines is known as Dihydropholate reductase. And the substrate to this enzyme is dihydrophobate. Now, we also have this molecule known as methotrexate. And methotrexate is a competitive inhibitor to this substrate to this enzyme here. In fact, methotrexate is about 1000 times more likely to actually bind onto the active side of Dihydrofolate reductase than Dihydropholate itself.