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It has been proven that “side population (SP)” cells that exclude Hoechst 33342 dye are enriched with cancer stem cells in several tumors. In the present study we aimed to isolate and characterize SP cells from human primary osteosarcoma. Side population cells were detected in osteosarcoma samples. In vitro, SP cells regenerated both SP and non-SP and the clonogenicity of SP cells was higher than that of non-SP cells, just like stem cells. In vivo, SP cells exhibited heightened tumorigenicity and only the SP fraction had the capacity to self- renew both in vitro and in vivo. Furthermore, SP cells exhibited increased multidrug resistance and the RNA expression of ATP-binding cassette protein transporters was increased in the SP group. In addition, “stemness” genes Oct-4 and Nanog were also upregulated in the SP group. However, the expression of other putative stem cell markers (CD44, CD117 and CD133) had no significant difference between SP and non-SP for each individual marker. These findings suggest that SP cells derived from osteosarcoma are enriched with tumorigenic cells with stem-like properties and might be an ideal target for clinical therapy. (Cancer Sci 2011; 102: 1774–1781)