The methylation of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) is an essential step in the formation of thymine nucleotides (1,2, reviewed in 3). This process is catalyzed by thymidylate synthase (TS or TYMS), a homodimer composed of two 30kD subunits. TS is an intracellular enzyme that provides the sole de novo source of thymidylate, making it a required enzyme in DNA biosynthesis with activity highest in proliferating cells (1). Being the exclusive source of dTMP, TS is also an important target for anticancer agents such as 5-fluorouracil (5-FU) (1-5). 5-FU acts as a TS inhibitor and is active against solid tumors such as colon, breast, head, and neck. Studies have shown that patients with metastases expressing lower levels of TS have a higher response rate to treatment with 5-FU than patients with tumors that have increased levels of TS (5). TS expression may be useful in predicting overall patient survival, but the prognostic value of TS expression continues to be a subject of investigation for different types of cancers (6-10).