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 an RNA-binding protein or the roles it plays in ES cells and mice.