Thymidylate synthase (TYMS) is a folate-dependent enzyme that catalyzes the methylation of deoxyuridine-5’-monophosphate (dUMP) using 5,10-methylene-tetrahydrofolate (5,10-CH2-THF) as the methyl donor to form deoxythymidine-5’-monophosphate (dTMP). This function maintains the dTMP pool, which is critical for DNA replication and repair, which in turn is essential for cell proliferation. Furthermore, TYMS protein and mRNA levels are elevated in several tumors, and ectopic expression of catalytically active TYMS is able to induce a transformed phenotype in mammalian cells as manifested by foci formation, anchorage independent growth, hyperplasia and tumor formation in nude mice. These studies highlighted the role of TYMS as a transforming factor and a potential oncogene.