Epidemiologists often investigate connections between cancers and chemicals in the environment. But molecular biologists now are starting to describe the molecular mechanisms for how such compounds might promote cancerous growth. A recent study is among the first to show how carcinogens trigger cancer-causing genetic events (A. Bardelli et al., "Carcinogen-specific induction of genetic instability," Proceedings of the National Academy of Sciences, Early Edition, April 10, 2001, <www.pnas.org/cgi/content/abstract/081082898v1). Johns Hopkins University cancer researchers treated genetically stable human cells with two carcinogens found in food: PhiP (2-amino-1-methyl-6-phenylimidazo [4,5-b] pyrdine) and MNNG (methylating agent methyl-N'-nitrosoguanine). PhiP is found typically in well-cooked beef and chicken; MNNG is produced in the colon during food metabolism. The researchers found that only cells that had acquired a mutation in a mismatch repair gene survived MNNG exposure, but the survivors were not able to efficiently repair DNA replication mistakes--a defect that can lead to cancer....

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