 Thymidylate synthaseTS is a critical therapeutic target for the fluoropyrimidine cytotoxic drugs that are the mainstay of the treatment for patients with advanced colorectal cancer. TS provide the sole de nono source of thymidylate for DNA synthesis. The clinical importance of TS in determining fluoropyrimidine cytotoxicity has been established in both clinical and preclinical study. Increased expression of TS protein due to underlying gene amplification has been described in cell lines selected for drug resistance by exposure to 5-FU.22,23 Several investigators have also demonstrated that intratumoral TS activity are predictive for response to 5-FU.24-26 High TS activity was associated with no response, whereas a low TS activity was associated with good response to 5-FU. 