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Dihydrofolate reductase (DHFR). Reduces 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate with NADPH as a cofactor. This is an essential step in the biosynthesis of deoxythymidine phosphate since 5,6,7,8-tetrahydrofolate is required to regenerate 5,10-methylenetetrahydrofolate which is then utilized by thymidylate synthase. Inhibition of DHFR interrupts thymidilate synthesis and DNA replication, inhibitors of DHFR (such as Methotrexate) are used in cancer chemotherapy. 5,6,7,8-tetrahydrofolate also is involved in glycine, serine, and threonine metabolism and aminoacyl-tRNA biosynthesis.