Embryonic stem (ES)-like cells can be induced from either mouse embryonic or adult fibroblasts by introducing four factors (Oct3/4, Sox2, c-Myc, and Klf4) and by selection for Fbx15 expression. These Fbx15 PS (induced pluripotent stem) cells are similar to ES cells in their morphology, proliferation and teratoma formation. Fbx15 PS cells, however, showed a different gene expression pattern from ES cells and they failed to produce adult chimeras. More ES-like PS cells might be generated by using a more ES specific marker than Fbx15. Based on this hypothesis, we focused on Nanog in this study, because of its essential role in pluripotency. Embryonic fibroblasts (MEF) were isolated from Nanog reporter mice, and Nanog PS cells were established by retrovirus-mediated transfection of the aforementioned four factors. Nanog PS cells showed morphology, proliferation, and teratoma formation similar to those of ES cells. These Nanog PS cells highly expressed ES cell markers, such as Nanog, Fbx15, ERas, and ESG1. Transgene expression of the four factors was strongly silenced in Nanog PS cells. When transplanted into blastocysts, adult chimeric mice were obtained from Nanog PS cells. Moreover, Nanog PS cells were transmitted through the germline to the next generation from the chimeric mice. Around 20% of the offspring, however, formed tumors, in which the reactivation of retroviral c-Myc was observed. These data demonstrated that germ-line competent PS cells can be obtained from MEF by the four factors and Nanog-selection. However, the retroviral transduction of c-Myc should be replaced by other methods prior to future medical application.