Background: MicroRNAs (miRs) are small (19-25 nucleotides), non-protein coding RNAs that regulate gene expression, and thus play essential roles in cell cycle progression. The evidence has demonstrated that the expression of several miRs is dysregulated in human cancer. Methods: The study includes 179 female patients and 58 healthy women Patients were identified as luminal A, B, Her-2/neu, and basal-like, as well as classified into I, II, and III stages. Analysis of the expression fold change of miR-21 and miR-34a with molecular markers, including the oncogene Bcl-2 (B-cell lymphoma 2) and the tumor suppressor genes BRCA1 (breast cancer susceptibility gene 1), BRCA2 (breast cancer susceptibility gene 2), and the tumor suppressor protein p53, was carried out for all patients, pre-and postchemotherapy, and for all healthy women. Results: At diagnosis (pre-chemotherapy), miR-21 was up-regulated (p < 0.001), while miR-34a was down-regulated (p < 0.001). Post-chemotherapy, the expression of miR-21 decreased significantly (p < 0.001), while the expression of miR-34a increased significantly (p < 0.001). Conclusion: miR-21 and miR-34a may be helpful to non-invasive biomarkers to evaluate the response of breast cancer to chemotherapy. AbbreviationsAUC Area under the curve BC Breast cancer Bcl-2 B-cell lymphoma 2 BRCA Breast cancer susceptibility gene DAB Diaminobenzidine ER Estrogen Receptor HER2 Human epidermal growth factor receptor 2 miR microRNA p53 Tumor suppressor protein p53 PR Progesterone receptor ROC Receiver operating characteristic UTR Untranslated region

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