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Cytochrome P4501B1 (CYP1B1) is involved in the activation of many carcinogens and in the metabolism of steroid hormones, including 17β-oestradiol (E 2) and testosterone. We report a significant difference in the allele frequencies of two point mutations in the coding region of the CYP1B1 gene among Caucasian (n= 189), African-American (n= 52) and Chinese (Linxian)(n= 109) populations. A (C to G) transversion at position 1666 in exon 3, which results in an amino acid substitution of Leu 432 to Val, was present in African-Americans with an allele frequency for Val 432 of 0.75, in Caucasians of 0.43, and in Chinese of 0.17. A (C to T) transition at position 1719 in exon 3, with no amino acid change (Asp 449), appeared to be closely linked with the Val 432 variant. Results using human lung microsomal preparations from individuals with the CYP1B1 Val/Val and CYP1B1 Leu/Leu genotypes indicate that Val 432 …