4012 Background: Exploratory biomarker analysis was conducted to identify factors related to outcomes of patients enrolled in the ACTS-GC study, a randomized controlled trial comparing adjuvant S-1 administration with surgery alone in 1,059 patients with stage II/III gastric cancer. Methods: Formalin-fixed paraffin-embedded surgical specimens were retrospectively examined in 829 patients (78.3%), and 63 genes involved in pyrimidine metabolic pathway, growth factor signaling pathway, apoptosis, DNA repair, etc., were analyzed by quantitative real-time RT-PCR after TaqMan assay-based pre-amplification. Gene expression levels were normalized to the geometric mean expressions of GAPDH, ACTB, and RPLP0, used as reference genes. The expression of each gene was categorized into low and high values at those median. The impacts of gene expression on survival were analyzed using the 5-year survival data of the ACTS-GC. The Benjamini and Hochberg procedure was used to control the false discovery rate (FDR). Results: Among 63 screened genes, IGF1R and AREG most strongly correlated with overall survival (OS), with FDR of 0.0048 and 0.018, respectively. OS was significantly worse in IGF1R high patients than in IGF1R low patients, but better in AREG high patients than in AREG low patients. The hazard ratio (HR) for death in the analysis of OS (S-1 vs. surgery alone) was lower in the high IGF1R group (HR, 0.55; 95%CI, 0.40-0.76) than in the low IGF1R group (HR, 0.72; 95%CI, 0.49-1.06). Likewise, the HR for death in the analysis of OS (S-1 vs. surgery alone) was much smaller in the low AREG group (HR, 0.57; 95%CI, 0.41-0.79) than in the high AREG group (HR, 0.74; 95%CI, 0.51-1.08). Interactions were not statistically significant. Conclusions: IGFR1 gene expression was associated with poor outcomes after curative resection of stage II/III gastric cancer, whereas AREG gene expression was associated with good outcomes. No interaction for survival was evident between S-1 and these gene expressions.