We recently established a novel tumor cell line denominated as F6, which was derived from mutated human embryonic bone marrow mesenchymal stem cells (MSCs). The difference between gene expression of F6 cells and MSCs was distinguished by fluorescent differential display. Results showed that the expression of nucleostemin, a novel factor participating in the control of stem and cancer cell proliferation, was different in F6 cells and MSCs. To further understand its role in transforming human embryonic bone marrow mesenchymal stem cells into F6 tumor cells, the full-length nucleostemin gene (1650 bp) from an LTEP-a-2 cell line was cloned, and GST-nucleostemin protein was expressed in E. coli. The characteristics of nucleostemin expression in F6 cells and other human cancer cell lines were investigated by RT-PCR, Western blot analysis, immunocytochemical staining, fluorescent microscope and confocal laser scanning microscope. The levels of nucleostemin gene expression were detected by real-time PCR in F6 tumor tissue obtained from SCID nude mice at 4, 6 and 7 weeks after the injection of F6 cells, and from the lung tissue of five lung cancer patients. Results showed that nucleostemin gene expression increased significantly in F6 tumor tissue and lung cancer tissue. The results also showed that transfection with pcDNA3.1(+)-GFP-nucleostemin for 4-20 weeks promoted cell size augmentation and nuclei multiplication, and the cells were converted to giant cell-like cells. Western blot analysis revealed that expression levels of nucleostemin in the nuclei and cytoplasm of cancer cell lines, e.g. F6, LTEP-a-2, U937, SW480 and 95D, were higher than those in MSCs and COS-7 cells. Levels of nucleostemin in F6 cells were notably high and confirmed with immunohistochemical staining. These results implied that nucleostemin may play an important role in both tumorigenesis and transforming human embryonic bone marrow mesenchymal stem cells into F6 tumor cells.