e22027 Background: Detection of circulating tumor cells (CTC) may provide diagnostic and prognostic information in breast cancer (bc) patients. Deregulation of microRNAs (miRNAs) is frequent in tumors, including breast cancer. It is suggested miRNAs play a role in tumor progression. We hypothesized that miRNAs may constitute a promising new class of cancer biomarkers for CTC detection. Our objective has been to identify miRNAs potentially useful for CTC detection. Methods: Phase I preclinical study was performed by means of computational tools for miRNAs profiling including MIRGATOR, MIRBASE, SMIRNAdb, GeneHUB-GEPIS, MICRORNA.ORG, and MIRNAMAP. In silico data were used to identify and prioritize miRNAs highly expressed in breast cancer, but absent in hematopoietic-derived sources. Selected miRNAs were evaluated by means of qRT-PCR in breast cancer and Hematopoietic cell lines, normal blood, and blood from breast cancer patients. Results: Computational tools identify a set of miRNAs highly expressed in breast cancer sources in relation to hematopoietic samples. Among these were miR-141, miR-200c, miR-196a, miR-203, miR-200a, miR-200b, miR-32, miR-375, miR-31, miR-193a, and miR-205. For instance, relative expression of miR-32 was 10⁴ higher in bc cell lines (N= 5) than in normal PB (N = 19) using qRT-PCR. In a preliminary analysis of PB from bc patients (stage IV) higher relative expression levels for selected miRNAs were found comparing with age-matched controls’ blood. Conclusions: Our results suggest that miRNA bioinformatic approach is a useful high-throughput method to select bc-associated miRNAS. The selected miRNAs should be further evaluated for their potential as markers for CTC detection. We next investigate if blood miRNA profile could predict tumor progression and survival. Supported by grants PI06/1541 and PI07/0477 from Fondo de Investigaciones Sanitarias (FIS), Instituto de Salud Carlos III. No significant financial relationships to disclose.