We are interested in the biology of embryonic stem (ES) cells, which have the ability to give rise to all cell types of the body. The pluripotent state of ES cells is maintained by a set of transcription factors including Oct4, Sox2 and Nanog. Some of these factors appear to be important also in other cell types such as neural stem cells as well as in different biological processes such as nuclear reprogramming and cancer. Based on these observations, we have recently generated pluripotent cells directly from fibroblasts by ectopic expression of the trancription factors Oct4, Sox2, c-myc and Klf4. These cells are indistinguishable from ES cells and provide a platform for analyzing the molecular mechanism of epigenetic reprogramming as well as a system to generate patient-specific stem cells for studying and treating disease.