Estrogen-induced carcinogenesis involves enhanced cell proliferation (promotion) and genotoxic effects (initiation).To investigate the contribution of estrogens and their metabolites to tumor initiation, we examined DNA damage induced by estradiol and its metabolites, the catechol estrogens 2-hydroxyestradiol (2-OHE 2 ) and 4-hydroxyestradiol (4-OHE 2 ). In the presence of Cu(II), catechol estrogens formed piperidine-labile sites at thymine and cytosine residues in 32 P 5 -end-labeled DNA fragments and induced the formation of 8-oxo-7, Epidemiological studies and animal experiments have revealed that estrogens are carcinogenic. 1 In humans, elevated circulating estrogen levels, caused either by increased endogenous hormone production or by therapeutic doses of estrogen medications, increase breast and uterine cancer risk. 2 Prolonged exposure of women to high estrogen levels is associated with an elevated incidence of breast cancer. 3 Administration of estradiol to mice increased the incidence of mammary, pituitary, uterine, cervical, vaginal, testicular, lymphoid and bone tumors. 2,4 In rats, estrogen increased the incidence of mammary and/or pituitary tumors. 2,5 Estrogens can cause cancer by stimulating cell proliferation and causing genotoxic damage. 6,7 Estrogens are highly mitogenic in hormone-sensitive tissues, such as the uterus and breast. 8 Prolonged exposure of target tissues and cells to excessive mitogenic stimulation by estrogens has been considered an important etiological factor for the induction of estrogen-associated cancers in experimental animals and humans. 8 Thus, estrogens are considered to participate in tumor promotion.However, estrogens have been reported to cause cancer through their genotoxic effects. Estradiol caused sister chromatid exchanges and chromosomal aberrations in peripheral blood human lymphocyte culture, and metabolic activation enhanced these chromosomal lesions. 9 Catechol estrogens are also capable of causing chromosomal aberrations and gene mutations in cultured cells. 10 Thus, the genotoxic effects of estradiol could result from accumulation of potentially carcinogenic metabolites. Estradiol undergoes cytochrome P-450 -catalyzed hydroxylation to various catechol estrogens. 8 2-Hydroxylation is a major metabolic pathway in the liver, whereas 4-hydroxylation dominates in extrahepatic organs susceptible to estrogens. 8,11,12 Catechol estrogens show much more potent carcinogenic effects on mice than estradiol. 13 Therefore, catechol estrogens can be potent carcinogens and may participate in tumor initiation by causing DNA damage.To investigate the role of catechol estrogens in tumor initiation, we examined DNA damage induced by 2-hydroxyestradiol (2-OHE 2 ) and 4-hydroxyestradiol (4-OHE 2 ) using 32 P 5Ј-end-labeled DNA fragments obtained from the human p53 tumor-suppressor gene and the c-Ha-ras-1 proto-oncogene. We also measured the formation of 8-oxo-7,8-dihydro-2Ј-deoxyguanosine (8-oxodG), an indicator of oxidative damage to DNA, using an electrochemical detector coup...