A common deletion polymorphism in the gene coding for the glutathione S-transferase class mu (the GSTM1 gene) results in a decreased ability to detoxify carcinogenic epoxide intermediates and has been associated with increased breast cancer risk in some small studies. We studied the GSTM1 gene deletion polymorphism (conferring the null genotype) in 243 women who had prevalent breast cancer and 245 women without breast cancer, who were among the 32,826 women in the Nurses' Health Study who gave a blood sample in 1989-1990. In the prevalent case series, the null genotype was slightly more common among cases (58%) than among controls (51%; age-adjusted odds ratio = 1.30; 95% confidence interval, 0.91-1.86). Among cases, the prevalence of the GSTM1 deletion increased with duration of survival [68% for > or = 8 years since diagnosis; 57% for 4-8 years; 51% for < 4 years; P (trend) = 0.04]. In an incident case series of 240 women who were diagnosed with breast cancer following blood collection and prior to June of 1992 and compared with age-matched controls, the GSTM1 deletion was not associated with an elevation in risk (relative risk, 1.08; 95% confidence interval, 0.74-1.57). No significant interaction with cigarette smoking was evident. Thus, there was no significant increase in risk of incident breast cancer associated with the GSTM1 null genotype; however, the gene deletion polymorphism appeared to confer improved survival. These data suggest that odds ratios based upon prevalent cases in molecular epidemiologic studies may be biased due to differential survival. Further studies are required to determine whether this polymorphism is associated with improved breast cancer prognosis.

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