S-1 is oral fluoropyrimidine preparation developed by Taiho Pharmaceutical Co., Ltd. (Tokyo, Japan) that combines tegafur with two 5-FU modulators, 5-chloro-2, 4-dihydroxypyridine (CDHP) and potassium oxonate (Oxo), in a molar ratio of 1:0.4:1. Tegafur, a prodrug of 5-FU, is converted to 5-FU mainly in liver and tumor cells. CDHP, a reversible inhibitor of dihydropyrimidine dehydrogenase, suppresses the degradation of 5-FU, thereby maintaining high concentrations of 5-FU in plasma and tumor cells. CDHP also decreases cardiotoxic and neurotoxic effects by reducing the production of F-b-alanine (FBAL), the main catabolite of 5-FU. Several phase II trials showed that TS-1 monotherapy or combination with CDDP, paclitaxel or irinotecan was effective palliative treatment option for advanced gastric cancer and colorectal cancer.