Groundbreaking work demonstrated that ectopic expression of four transcription factors, Oct4, Klf4, Sox2, and c-Myc, could reprogram murine somatic cells to induced pluripotent stem cells (iPSCs)(Takahashi and Yamanaka, 2006), and human iPSCs were subsequently generated using similar genetic manipulation (Takahashi et al., 2007; Yu et al., 2007). To address the safety issues arose from harboring integrated exogenous sequences in the target cell genome, a number of modified genetic methods have been developed and produced iPSCs with potentially reduced risks (for discussion, see Yamanaka, 2009, and references therein). However, all of the methods developed to date still involve the use of genetic materials and thus the potential for unexpected genetic modifications by the exogenous sequences in the target cells. Here we report generation of protein-induced pluripotent stem cells (piPSCs) from murine embryonic fibroblasts using recombinant cell-penetrating reprogramming proteins. We demonstrated that such piPSCs can long-term self-renew and are pluripotent in vitro and in vivo. One possible way to avoid introducing exogenous genetic modifications to target cells would be to deliver the reprogramming proteins directly into cells, rather than relying on the transcription from delivered genes. Previous studies have demonstrated that various proteins can be delivered into cells in vitro and in vivo by conjugating them with a short peptide that mediates protein transduction, such as HIV tat and poly-arginine (Inoue et al., 2006; Michiue et al., 2005; Wadia and Dowdy, 2002). In addition, various solubilization and refolding techniques for processing inclusion body proteins expressed in E. coli to bioactive proteins have been developed to allow facile and largescale production of therapeutic proteins (Lafevre-Bernt et al., 2008). To generate recombinant proteins that can penetrate across the plasma membrane of somatic cells, we designed and fused a poly-arginine (ie, 11R) protein transduction domain to the C terminus of four reprogramming factors: Oct4, Sox2, Klf4, and c-Myc (see