Background: The multifunctional mammalian protein YB-1 is a member of the large DNA-and RNA-binding protein family with an evolutionarily ancient cold-shock domain.YB-1 is involved in multiple DNA-and mRNAdependent events and regulates gene expression at various levels.YB-1 protein has pleiotropic functions in the regulation of gene transcription and translation, DNA repair, drug resistance, and cellular responses to environmental stimuli (Ovchinnikov LP et al.Mol Biol (Mosc).2001, 35:548-58).This protein could be found in the cytoplasm and the nuclei of mammalian cells.In some tumors YB-1 mRNA content is elevated (Vaiman A.et al, Biochemistry (Mosc).2006, 71:146-54).Our data showed that it is reasonable to apply the complex test of YB-1 determination (determination of YB-1 mRNA content and studies of YB-1 intercellular localization) for the studying of the groups of breast cancer patients (Vaiman A., Guens G.et al., Molecular Medicine, 2007, 1:31-37).Here we present the results of the testing breast cancer patients for YB-1 by means of this test..Patients and methods: Breast cancer patients stage Ⅰ-Ⅱ who underwent mastectomy were enrolled in this study.We used RT-PCR, immunohistochemistry, flow cytometry, Western blotting, siRNA technique.Results: Semi-quantitative RT-PCR method had revealed various quantities of mRNA in breast cancer specimens.Large quantity of YB-1 mRNA correlated with the reduction of free-time to relapse, with clinical evidence of disease progression (distant metastasis) and with smaller tumor size (T1-T2).This data suggest that metastasizing capacity of small tumors is connected with YB-1 mRNA quantity.When YB-1 was localized in the nuclei of cancer cells then more aggressive tumors "behavior" in breast cancers patients was observed.Nuclear localization of YB-1 correlated with lager tumor size (T3-T4).We suppose that intercellular YB-1 localization and the quantity of YB-1 mRNA are separate tumor characteristics with different predictive capacity.