DDX4 (or VASA) is an intracytoplasmic, ATP-dependent RNA helicase which is reported to be exclusively expressed by germ cells: expression in pluripotent cells and non-gonadal tissue has not been demonstrated (Castrillon et al., 2000, Toyooka et al., 2000). In both mice and humans, it is first detected in PGCs colonising the genital ridges and continues to be expressed throughout the rest of the germ cell developmental pathway, with expression persisting in post-meiotic oocytes (Castrillon et al., 2000, Toyooka et al., 2000, Stoop et al., 2005). In humans, its expression increases as the oocyte matures and, as with C-KIT, its initial co-localisation with POU5F1 ceases once folliculogenesis commences (Castrillon et al., 2000, Stoop et al., 2005). Bovine studies have not been as detailed, but demonstrate that DDX4 expression is constrained to germ cells in the fetus (Bartholomew and Parks, 2007) and can be detected up to the antral follicle stage in adult ovaries (Pennetier et al., 2004). In vitro co-culture studies have suggested that its expression is “switched on” by cell-cell interactions with somatic cells within the genital ridge (Toyooka et al., 2000) and its role in germ cell development appears to be relatively conserved across species, including C. elegans, Drosophila and mice, with knock-out studies demonstrating various germline deficiencies (Ewen-Campen et al., 2010). DDX4 is thus a well-established germ cell-specific marker across the phylogenetic spectrum (Ewen-Campen et al., 2010).