a-Difluoromethylornithine (DFMO)Osteogenic differentiation Small interfering RNA (siRNA) Mesenchymal stem cell a b s t r a c t Ornithine decarboxylase (ODC) is the rate-limiting enzyme for polyamine biosynthesis. Suppression of ODC by its irreversible inhibitor, a-difluoromethylornithine (DFMO), or by RNA interference through siRNA, enhanced osteogenic gene expression and alkaline phosphatase activity, and accelerated matrix mineralization of human bone marrow-derived mesenchymal stem cells (hBMSCs). Besides, adipogenic gene expression and lipid accumulation was attenuated, indicating that the enhanced osteogenesis was accompanied by down-regulation of adipogenesis when ODC was suppressed. A decrease in the intracellular polyamine content of hBMSCs during osteogenic induction was observed, suggesting that the level of endogenous polyamines is regulated during differentiation of hBMSCs. This study elucidates the role of polyamine metabolism in the lineage commitment of stem cells and provides a potential new indication for DFMO as bone-stimulating drug.