S1, an oral fluoropyrimidine composed of tegafur (a 5-FU prodrug), gimeracil (a dihydropyrimidine dehydrogenase, DPD, inhibitor), and potassium oxonate, is employed in Asia and in some European Countries for the treatment of different kinds of solid tumors, including advanced colorectal cancer. The gene discussed is DPYD; the disease is colorectal cancer.