This retrospective study aimed to evaluate the association of MSI and MMR deficiency with outcome and with thymidylate synthase (TS) and dihydropyrimidine dehydrogenase (DPD) expression in tumors from 340 consecutive patients who were completely resected for colorectal cancer stage II-IV and subsequently received adjuvant 5-FU treatment. The gene discussed is DPYD; the disease is mismatch repair cancer syndrome 1.