Embryonic stem cells have been established from mammalian blastocysts,,. ESCs have the ability to proliferate vigorously and differentiate into various cell types. Therefore, they are attractive sources for cell transplantation therapy and basic research. ESCs have been used for functional analyses of numerous genes and differentiation processes. Recently, induced pluripotent stem cells were derived from mouse and human somatic cells that have similar differentiation potential to ESCs, and can overcome the ethical problems and immune rejection associated with ESCs,,. The molecular mechanisms and pathways underlying the Aminoguanidine hemisulfate salt pluripotency and proliferation of ESCs and iPSCs are still unclear. In mouse ESCs, pluripotency can be maintained by leukemia inhibitory factor and several transcription factors. LIF activates Stat3 signaling and its downstream cascades that are Acepromazine maleate involved in pluripotency. Oct4, Sox2 and Nanog, are also pivotal regulators, and maintain the undifferentiated state of ESCs. Klf4 is also an important factor for the maintenance of ESCs. The Kluppel-like factor family, involving Klf4, Klf2 and Klf5, regulates the self-renewal of ESCs. Therefore, pluripotency is maintained by the regulatory networks of many transcription and other factors. To identify new genes involved in the molecular network of pluripotency, we have previously performed a digital differential display analysis of the expressed sequence tag libraries among various mouse tissues and cell lines,,,,,. Candidates were selected based on their specific expression in ESCs, and included many well-known pluripotency related genes, such as Oct4 and Nanog, as well as a variety of novel genes which we designated the ����ECATs���� for ES cell-associated transcripts. We have shown that ECAT4 encodes the transcription factor Nanog, which plays critical roles in pluripotency, whereas ECAT5 encodes Eras, which promotes the proliferation of mouse ESCs. In this study, we evaluated the expression and function of another ECAT, ECAT11, also known as L1ltd1. Wegenerated ECAT11 knock-outmice and ES cells by inserting the enhanced green fluorescent gene cDNA into the ECAT11 locus. Our study showed that ECAT11 is dispensable for the development and maintenance of pluriptotency, despite its specific expression pattern. We also found that ECAT11 is rapidly activated by Oct3/4, Sox2 and Klf4 in fibroblasts, but is dispensable for the generation of iPSCs.