IFITM3, also known as Fragilis, is a cell surface protein which is evolutionarily conserved, with homologues detected in the human and bovine genomes (Saitou et al., 2002, Lange et al., 2003). It was first identified in mouse embryos, with strong, specific expression seen in the area where PGCs specification occurs (Saitou et al., 2002). There are conflicting data regarding its role in migratory PGCs, with some research suggesting it is present (Tanaka and Matsui, 2002) and others reporting that it is markedly downregulated (Saitou et al., 2002), with knock-out studies demonstrating that Ifitm3 is not necessary for normal germ cell development, including migration (Lange et al., 2008). A possible role for Ifitm3 in PGC specification has been demonstrated, however, by examining the relationship between Bmp4 and Ifitm3 expression, with Bmp4-null mutants exhibiting no Ifitm3 expression (Saitou et al., 2002). With regards the timing of its expression, Iftim3 appears to precede that of Dppa3, with the latter only expressed in early PGCs with high levels of Ifitm3 (Saitou et al., 2002).

Human data are lacking, although bovine studies have demonstrated that IFITM3 expression is confined to the epiblast of the blastocyst, from which PGCs are specified (Smith et al., 2007). Given its conservation across species, however, it is likely that IFITM3 is a reliable marker of germ cells in humans and cows.