creasing. Important advances have increased our understanding of the factors that influence cancer risk, explaining in part why some cancer rates are increasing while others are decreasing, and guiding us closer to the means to reverse adverse trends.

A growing body of knowledge dramatically illustrates the influence of the environment, genes, and their interactions in the international variation reported in cancer incidence. A variety of linkages clearly exist between environmental exposures, diet, lifestyle factors, and cancer. Genetic factors also are known to be involved in the predisposition to and development of some cancers. Recent progress in identifying and characterizing highly penetrant susceptibility genes in familial cancer has revolutionized our understanding of the critical genetic mechanisms in cancer etiology. Studies that combine genetic analysis with assessment of exposures and diet can explain why not everyone exposed to a particular cancer-causing chemical will develop cancer. Genetic research also is shedding light on why some cancer patients respond to therapy and others do not.

The interactions of multiple modifier genes with various environmental factors—that is, gene–environment interactions—explain why cancer rates vary across populations, among exposed groups, and even within families. The research community is now studying cancer with an expanded and enhanced view of environmental health and exposures that include factors such as diet, lifestyle, metabolic alterations, socioeconomic status, and various environmental exposures.