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Cancer Risk for Twins
A Swedish study of female twins compared identical twins (who share all their genes) with fraternal twins (who share about half their genes) to determine the extent to which differences in breast cancer incidence could be attributed to differences in genetic makeup. The study found that genetic differences played a relatively minor role in breast cancer risk, suggesting that most differences among individuals in breast cancer risk was related to environmental factors. Even among identical twins, cancer in one twin usually was not accompanied by cancer in the other [Ahlbom et al.,1997].