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Objectives: Aromatase inhibitors that block estrogen synthesis are a proven first-line hor¬monal therapy for postmenopausal breast cancer. Although it is known that standard¬ized extract of Ginkgo biloba (EGb761) induces anti-carcinogenic effects like the aroma¬tase inhibitors, the effects of EGb761 on steroidogenesis have not been studied yet. Therefore, the effects of EGb761 on steroidogenesis and aromatase activity was studied using a H295R cell model, which was a good in vitro model to predict effects on human adrenal steroidogenesis. Methods: Cortisol, aldosterone, testosterone, and 17β-estradiol were evaluated in the H295R cells by competitive enzyme-linked immunospecific assay after exposure to EGb761. Real-time polymerase chain reaction were performed to evaluate effects on critical genes in steroid hormone production, specifically cytochrome P450 (CYP11/ 17/19/21) and the hydroxysteroid dehydrogenases (3β-HSD2 and 17β-HSD1/4). Finally, aromatase activi¬ties were measured with a tritiated water-release assay and by western blotting analysis. Results: H295R cells exposed to EGb761 (10 and 100 μg/mL) showed a significant de¬crease in 17β-estradiol and testosterone, but no change in aldosterone or cortisol. Genes (CYP19 and 17β-HSD1) related to the estrogen steroidogenesis were significantly de¬creased by EGb761. EGb761 treatment of H295R cells resulted in a significant decrease of aromatase activity as measured by the direct and indirect assays. The coding se¬quence/Exon PII of CYP19 gene transcript and protein level of CYP19 were significantly decreased by EGb761. Conclusions: These results suggest that EGb761 could regulate steroidogenesis-related genes such as CYP19 and 17β-HSD1, and lead to a decrease in 17β-estradiol and testos¬terone. The present study provides good information on potential therapeutic effects of EGb761 on estrogen dependent breast cancer.