Dihydrofolate reductase (DHFR) is an enzyme that catalyzes the reduction of dihydrofolate (H2F) to tetrahydrofolate (H4F) utilizing nicotinamide adenine dinucleotide phosphate (NADPH) as the hydride (H-) donor, in which the pro-R hydrogen of NADPH is transferred to C6 of the pteridine ring with concomitant protonation at the N5 position. The functional DHFR gene (approximately 30 kb with six exons and five introns) has been identified to encode DHFR enzyme. Human DHFR enzyme is a monomeric protein of 186 amino acids, which is characterized by containing a highly conserved dinucleotide-binding domain consisting of seven parallel and one antiparallel β-strands intercalated by α-helices.