Thymidylate synthase (TS) (EC 2.1.1.45) is an enzyme that catalyzes the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP). Thymidine is one of the nucleotides in DNA. With inhibition of TS, an imbalance of deoxynucleotides and increased levels of dUMP arise. Both cause DNA damage. By means of reductive methylation, deoxyuridine monophosphate (dUMP) and N5,N10-methylene tetrahydrofolate are together used to form dTMP, yielding dihydrofolate as a secondary product. This provides the sole de novo pathway for production of dTMP and is the only enzyme in folate metabolism in which the 5,10-methylenetetrahydrofolate is oxidised during one-carbon transfer. The enzyme is essential for regulating the balanced supply of the 4 DNA precursors in normal DNA replication: defects in the enzyme activity affecting the regulation process cause various biological and genetic abnormalities, such as thymineless death.