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The cellular metabolism of doxorubicin generates reactive oxygen species with significant potential to damage DNA. Such DNA damage can result in mutations if not adequately repaired by cellular DNA repair pathways. Secondary malignancies have been reported in patients who have received doxorubicin-containing chemotherapeutic regimens; however, the underlying molecular mechanism(s) to explain the development of these tumors remains under active investigation. We have previously demonstrated the presence of DNA bases modified by oxidation in the peripheral blood mononuclear cells of patients with breast cancer following treatment with doxorubicin. In those studies, doxorubicin was administered by continuous infusion over 96 h to minimize the risk of cardiac toxicity. To evaluate potential mechanisms underlying doxorubicin-induced DNA base oxidation in non-malignant tissues, MCF-10A breast epithelial cells were cultured for 96 h with the same doxorubicin concentration achieved in vivo (0.1 μM). During doxorubicin exposure, MCF-10A cells underwent growth arrest and apoptosis, developed elevated levels of reactive oxygen species, and demonstrated a time-dependent and significant increase in the levels of 11 oxidized DNA bases, as determined by gas chromatography/mass spectroscopy. Diminished expression of DNA repair enzymes was also observed over the same time course. Thus, clinically achievable concentrations of doxorubicin induce a level of oxidative stress in MCF-10A cells that is capable of oxidizing DNA bases and significantly altering cellular proliferation.