To assess the effects of 5-FU and FdUrd on the ATM and ATR checkpoint signaling pathways, we compared the abilities of these agents to activate checkpoint signaling in two colon cancer cell lines: HT29, which have a functional MMR system, and HCT-8, which have a mutations in MSH6 and are MMR deficient [58]. Here, ATM is linked to malignant colon neoplasm.