Unlike the chicken-and-egg dilemma, scientists know that genes come before the environment. In genetically based diseases, however, the question is not when but how. How do environmental conditions and lifestyle choices interact with a person's molecular structure? "We're all groping in the dark to find out how to get a handle on this complexity," says Marianne Berwick, an epidemiologist at Memorial Sloan-Kettering Cancer Center, New York.

Although scientists can now assess exposure levels to toxic agents and examine the body's response to those toxins through the presence of DNA variations known as biomarkers, finding the trigger point, or points, for disease onset is far more difficult. "Pure, genetically caused disease represents five to 10 percent of the disease burden in the United States; the rest is linked to a combination of multiple factors, both genetic and environmental," says Samuel Wilson, deputy department head, National Institute of Environmental...

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