This current phase II study of S-1/LV is deemed necessary to explore its efficacy and safety as first-line chemotherapy for AGC patients. Considering the frequent grade 3 toxicities, dose reduction, and period prolongation in that previous trial of mCRC , the 4-week S-1/LV was modified as 2 week in this study. As the first one, this study used S-1 pharmacogenetic pathway approach and integrated CYP2A6 polymorphism, DPD, TS, OPRT, thymidine phosphorylase (TP), and 5-FU pharmacokinetics to identify the subset of patients benefiting more, suffering less from S-1/LV.