Induced pluripotent stem (iPS) cells have been generated from mouse and human fibroblasts by the retroviral transduction of four transcription factors. However, the cell origins and molecular mechanisms of iPS cell induction remain elusive. This report describes the generation of iPS cells from adult mouse hepatocytes and gastric epithelial cells. These iPS cell clones appear to be equivalent to embryonic stem cells in gene expression and are competent to generate germline chimeras. Genetic lineage tracings show that liver-derived iPS cells are derived from albumin-expressing cells. No common retroviral integration sites are found among multiple clones. These data suggest that iPS cells are generated by direct reprogramming of lineage-committed somatic cells and that retroviral integration into specific sites is not required.