 Thymidylate synthase is an enzyme of about 30 to 35 kDa in most species except in protozoan and plants where it exists as a bifunctional enzyme that includes a dihydrofolate reductase domain.A cysteine residue is involved in the catalytic mechanism (it covalently binds the 5,6-dihydro-dUMP intermediate).The sequence around the active site of this enzyme is conserved from phages to vertebrates.