Analyses of the effects of the MMR pathway on cell viability following treatment with 5-FU and FdUrd have generally indicated that cells with defective MMR are more resistant to 5-FU and FdUrd [10], [36]–[38], a result consistent with the finding that MMR-defective colon cancer patients do not benefit from 5-FU therapy [39]. The gene discussed is MRC1; the disease is malignant colon neoplasm.