Thymidylate synthase (TS) is an enzyme catalyzing the sole de novo pathway for the biosynthesis of deoxythymidine monophosphate (dTMP) in which deoxyuridine monophosphate (dUMP) is reductively methylated by 5,10-methylenetetrahydrofolate (CH2H4folate) with the generation of dTMP and dihydrofolate (H2folate). TS has been found in a wide range of sources, including bacteria, bacteriophage, yeast, viruses and vertebrates, where it is usually a dimer consisting of identical subunits (30 to 35 kDa each). However, protozoal TS is a bifunctional protein with dihydrofolate reductase (DHFR) on the same polypeptide chain, where TS and DHFR domains are located on the carboxy terminus and amino terminus respectively.