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The CYP11A gene encodes the cholesterol side-chain cleavage enzyme (P450scc) that catalyzes the first and rate-limiting step for the biosynthesis of sex hormones. A pentanucleotide repeat [(TAAAA)n] polymorphism in the 5' of the CYP11A gene has been reported to be related to the risk of polycystic ovary syndrome, an inherited endocrine disorder characterized by hyperandrogenemia. We investigated the association of this polymorphism with breast cancer risk in a population-based case-control study conducted among Chinese women in Shanghai. Genotype assays were completed for 1015 incident breast cancer cases and 1082 community controls. Three common alleles with 4, 6, or 8 TAAAA repeats were identified in the study population. The frequency of the 8 repeat allele was more common in cases (12.6%) than controls (8.5%) (odds ratio = 1.6, 95% confidence interval = 1.3-1.9; P < 0.0001). Compared to subjects who did not carry this allele, adjusted odds ratios were 1.5 (95% confidence interval = 1.2-1.9) and 2.9 (1.3-6.7) (P for trend, <0.001), respectively, for those who carried one and two copies of this allele. This positive association was observed in both pre- and postmenopausal women and all strata defined by major breast cancer risk factors, including years of menstruation, body mass index, and waist-to-hip ratio. The results from this study indicate that the TAAAA repeat polymorphism near the promoter region of the CYP11A gene may be an important susceptibility factor for breast cancer risk.