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A key enzyme involved in the production of potentially carcinogenic estrogen metabolites and the activation of environmental carcinogens is cytochrome P450 1B1 (CYP1B1), the predominant member of the CYP1 family expressed in normal breast tissue and breast cancer. Because of the preeminent role of CYP1B1 in mammary estrogen/carcinogen metabolism, we examined the CYP1B1 gene to determine whether genetic differences could account for interindividual differences in breast cancer risk. We focused on exon 3, because it encodes the catalytically important heme binding domain of the enzyme, and discovered three polymorphisms of which two are associated with amino acid substitutions in codons 432 (Val-->Leu) and 453 (Asn-->Ser), designated as m1 and m2, respectively. Approximately 40% of Caucasian women have the m1 Val allele compared with nearly 70% of African-American women (P < 0.0001). The allele frequency also differs significantly in m2, with the rare Ser allele being present in 17.4% of Caucasians but only in 3.4% of African Americans (P < 0.0003). To determine whether the polymorphic CYP1B1 alleles hold implications as potential breast cancer risk factors, we compared the CYP1B1 genotypes in 164 Caucasian and 59 African-American breast cancer cases with those in age-, race-, and frequency-matched controls. Odds ratio calculations failed to show a significant association between any of the genotypes and breast cancer. Because CYP1B1 is known to be involved in mammary estrogen metabolism, we investigated whether the estrogen receptor status is influenced by the CYP1B1 genotypes. Caucasian patients with the m1 Val/Val genotype have a significantly higher percentage of estrogen receptor-positive (P = 0.02) and progesterone receptor-positive breast cancers (P = 0.003). There was no correlation with the m2 genotypes. These data suggest that the CYP1B1 polymorphisms in exon 3 are not associated with increased breast cancer risk but that the m1 polymorphism may be functionally important for steroid receptor expression in breast cancer of Caucasian patients.