Dihydrofolate Reductase (DHFR; 5,6,7,8-tetrahydrofolate NADP oxidoreductase; EC 1.5.1.3), is a ubiquitous enzyme that is present in all eukaryotic and prokaryotic cells. It catalyzes the reduction of dihydrofolate (FH2) to tetrahydrofolate (FH4) using NADPH as a cofactor. FH4 is essential for a number of enzymes that are necessary for the de novo synthesis of purines, thymidylic acid and some amino acids. Inactivation of the DHFR enzymatic activity causes reduction of the intracellular level of FH4, inhibition of RNA and DNA synthesis, and cell death. For this reason, DHFR has been a critically important enzyme as a molecular target in drug discovery. Biovision's Dihydrofolate Reductase assay kit is based on the ability of DHFR to catalyze