Background The contribution of hereditary factors to the causation of sporadic cancer is unclear. Stud- ies of twins make it possible to estimate the overall contribution of inherited genes to the development of malignant diseases. Methods We combined data on 44,788 pairs of twins listed in the Swedish, Danish, and Finnish twin registries in order to assess the risks of cancer at 28 anatomical sites for the twins of persons with cancer. Statistical modeling was used to estimate the rela- tive importance of heritable and environmental fac- tors in causing cancer at 11 of those sites. Results At least one cancer occurred in 10,803 persons among 9512 pairs of twins. An increased risk was found among the twins of affected persons for stomach, colorectal, lung, breast, and prostate cancer. Statistically significant effects of heritable factors were observed for prostate cancer (42 percent of the risk may be explained by heritable factors; 95 percent confidence interval, 29 to 50 percent), colorectal can- cer (35 percent; 95 percent confidence interval, 10 to 48 percent), and breast cancer (27 percent; 95 per- cent confidence interval, 4 to 41 percent). Conclusions Inherited genetic factors make a mi- nor contribution to susceptibility to most types of neoplasms. This finding indicates that the environ- ment has the principal role in causing sporadic can- cer. The relatively large effect of heritability in cancer at a few sites (such as prostate and colorectal can- cer) suggests major gaps in our knowledge of the genetics of cancer.
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