Thymidylate synthase (TS; N 5 ,N 10 -methylenetetrahydrofolate: dUMP C-methyltransferase; EC 2.1.1.45) catalyzes the conversion of deoxyuridylate to thymidylate, and is a key enzyme that controls a balanced supply of four DNA precursors. The gene for TS is a typical housekeeping gene and its expression is regulated depending on both the cell cycle [1][2][3] and the state of cell proliferation.4-7) The human TS gene and its cDNA were isolated [8][9][10][11] and their primary and functional structures have been extensively elucidated.