Objective-The introduction of 4 transcription factors-c-MYC, OCT3/4, SOX2, and KLF4 -can reprogram somatic cells back to pluripotency. However, some of the factors used are oncogenic, making therapeutic application unfeasible. Although the use of adult stem cells expressing high endogenous levels of some of these factors allows for reprogramming with fewer exogenous genes, such cells are rare and may have accumulated genetic mutations. Our goal was to reprogram human somatic cells without oncogenic factors. We found that high endogenous expression of KLF4 in human umbilical vein endothelial cells (HUVECs) allows for generation of induced pluripotent stem cells (iPSCs) with just 2 nononcogenic factors, OCT3/4 and SOX2. Methods and Results-HUVECs were infected with lentivirus containing OCT4 and SOX2 for generation of iPSCs. These 2-factor HUVEC iPSCs were morphologically similar to embryonic stem cells, express endogenous pluripotency markers postreprogramming, and can differentiate toward lineages of all 3 germ layers both in vitro and in vivo. T he discovery that somatic cells can be reprogrammed to pluripotency with the addition of 4 transcription factorsc-Myc, Oct3/4, Sox2, and Klf4 -is revolutionary. 1,2 These induced pluripotent stem cells (iPSCs) are similar to embryonic stem cells but can be generated without ethical concern and transplanted without immune rejection. Although this method of reprogramming is straightforward, efficiency is still quite low even in the best system. 3 Subsequent reports reveal that oncogenic factors increase efficiency, 4 but in anticipation of clinical use, exclusion of such factors is necessary. Conclusion-iPSCs See accompanying article on page 1880To date, the generation of human iPSCs from adult somatic cells has been reported 5,6 but generally with 3 or more factors, because reprogramming of human cells is less efficient than reprogramming of mouse cells 3 and may require immortalization in some instances. 6 Recent reports have shown that some human adult stem cells can be reprogrammed with relatively few factors because of endogenous expression of SOX2. 7 However, the rarity of adult stem cells makes this a difficult option. Because fetal-derived cells (FDCs) are more proliferative and at an earlier stage of development than adult cells, we hypothesized that reprogramming of FDCs may require fewer factors and obviate oncogenes. As proof of principle, we used an easily obtainable source of FDCs for iPSC generation: human umbilical vein endothelial cell (HUVECs). We found that HUVECs endogenously express high levels of KLF4 and can be reprogrammed with just OCT3/4 and SOX2. Our data may have important implications in terms of providing an efficient in vitro model for human iPSCs research and possibly open up a new cell source for reprogramming. MethodsHUVECs (Bioresource Collection and Research Center, Hsinchu, Taiwan) were infected with lentivirus containing OCT3/4 and SOX2 and monitored for iPSC formation as previously reported. 1 Reprogrammed HUVECs were assayed for in...