Methylating agents of the SN1 type represent an important class of cancer chemotherapeutics. Efficient killing by clinically-relevant doses of these agents requires cell division and low levels or absence of the repair enzyme methylguanine methyl transferase (MGMT). The process requires also an active mismatch repair (MMR) system, as treatment of cells with the prototypic methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) could be shown to trigger a delayed cell cycle arrest, which was absolutely MMR-dependent. We now show that DNA damage signaling activated by high doses of MNNG is very rapid and largely MMR-independent. However, the MMR system still contributes towards cell killing, as MMR deficiency favors the long-term survival of the cells, albeit to a substantially smaller extent than when low MNNG concentrations are deployed.