24, 25 DNA-alkylating agents induce phosphorylation and activation of p53, leading to an increased expression of p21waf1/cip1. Cell lines with MMR deficiency are resistant to these alkylating agents and bypass the cell cycle arrest, indicating that the MMR has a role in post-replication checkpoints.26, 27 However, nitric oxide (NO) and H2O2 are capable of arresting hMLH1 mutant cells in G2/M.4, 28 No information exists on the role of hMSH2 in mediating such a cell cycle arrest. The gene discussed is MRC1; the disease is hyperinsulinemic hypoglycemia, familial, 4.