focus of the field of stem cell biology [4-6].
To identify molecules essential in ES cells for these properties, several groups have utilized transcriptome analyses to identify genes specifically expressed in ES cells and early embryos. These analyses, including DNA microarray analyses [7] and expressed sequence tag analyses [8-12], identified several common transcripts, including ESG1 that was also designated dppa5 or ECAT2.
ESG1 was originally identified as a transcript Ph34 that was down-regulated by retinoic acid in embryonic carcinoma cells [13]. The expression of this gene was confined in mice to early embryos and germ cells [14]. It is also expressed in pluripotent cells, including ES cells, embryonic germ cells, and multipotent germline stem cells [15]. ESG1 encodes a polypeptide of 118 amino acids that contains a single KH domain, which is an RNA-binding domain [16]. It remains unclear, however, if ESG1 functions as