The literature shows severe diarrhea was dose-limiting toxicity for S-1 in Caucasians and severe neutropenia in East Asians perhaps due to CYP2A6 gene polymorphism [17], while this study did not see its relation to severe diarrhea, neutropenia, or total grade 3–4 AEs and no relation of CYP2A6 gene polymorphism to AUC0–24h or Cmax of 5-FU here helped explain. This evidence concerns the gene CYP2A6 and Decreased total neutrophil count.