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Platelets are implicated in the pathophysiology of breast and other cancers through their role in exchanging biomolecules with tumor cells in the tumor microenvironment. Such exchange results in tumor‐educated platelets with altered RNA expression profiles. Multiple lines of evidence indicate that platelet RNA profiles may be suitable as diagnostic biomarkers for cancer‐related biological processes. In this study, we characterized the gene expression signatures of platelets in breast cancer (BC) by high‐throughput sequencing and quantitative real‐time RT‐PCR. Our results indicate that the expression of TPM3 (tropomyosin 3) mRNA is significantly elevated in platelets from patients with BC compared with age‐matched healthy control subjects. Furthermore, up‐regulation of TPM3 mRNA in platelets was found to be significantly correlated with metastasis in patients with BC. Finally, we report that platelet TPM3 mRNA is delivered into BC cells through microvesicles and leads to enhanced migrative phenotype of BC cells. In summary, our findings suggest that the transfer of platelet TPM3 mRNA into cancer cells via microvesicles promotes cancer cell migration, and thus platelet‐derived TPM3 mRNA may be a suitable biomarker for early diagnosis of metastatic BC. [ABSTRACT FROM AUTHOR]