TS (EC:18.104.22.168), a cytosolic enzyme, is a dimmer of two identical monomers of about 36kDa. The enzyme provides the sole intracellular de novo source of thymidylate and plays a crucial role in DNA replication and repair. TS catalyzes the methylation of deoxyuridine monophosphate (dump) and its conversion to deoxythymidine monophosphate (dTMP). Therefore, TS is primarily active in proliferating and metabolic active cells. TS is a central target of the widely used antineoplastic agent 5-Fluorouracil (5-FU) and thus also of the Xeloda, which is enzymatically activated to 5-FU. TS is inactivated by a covalent complex formation with 5-FdUMP and methylenetetrahydrofolate. Literature indicates that expression of TS is associated with response to 5-fluorouracil (5-FU) in human breast, colorectal, gastric, head, and neck carcinomas with low TS expression predicting better response to 5-FU and survival.