Dihydrofolate reductase and methotrexate. Computer model showing the structure of a human dihydrofolate reductase molecule (ribbons) bound to its cofactor nicotinamide adenine dinucleotide phosphate (NADPH, ball-and-sticks) and methotrexate (spheres). Dihydrofolate reductase is an enzyme that converts folic acid into a coenzyme. Methotrexate inhibits DHFR and is used in the treatment of cancer, autoimmune diseases and ectopic pregnancies.