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Background: Human mesenchymal stem cells (hMSCs) hold promise for use in cell-based therapies and tissue engineering. Although hMSCs are thought to be stable ex vivo, it is possible that they undergo an undesirable transformation to a phenotype of unlimited proliferation during ex vivo. In this study, we searched for the factor required for unlimited proliferation of hMSCs. Methods: Changes in gene expression were evaluated between hMSCs and Ewing’s sarcoma cell lines, which may be derived from hMSCs, using GeneChip Human Genome U133 plus 2.0 Array. A gene up-regulated by at least 10-fold in Ewing’s sarcoma cell lines, Cyclin D2, was overexpressed in hMSCs by a lentiviral vector. Results: Overexpression of Cyclin D2 in hMSCs altered cell morphology and promoted cell proliferation. Expression of transforming growth factor-b2 (TGF-b2), which induces senescence in hMSCs, was down-regulated in Cyclin D2- overexpressing hMSCs. Furthermore, Gene Ontology analysis revealed that Cyclin D2 overexpression activated expression of genes associated with proliferation and interphase. Conclusions: Cyclin D2 promotes hMSC proliferation and is a candidate biomaker for hMSC transformation.