Molecular mechanisms underlying the commitment of cells to the germ cell lineage during mammalian embryogenesis remain poorly understood due to the limited availability of cellular materials to conduct in vitro analyses. Although primordial germ cells (PGCs)--precursors to germ cells--have been generated from embryonic stem cells (ESCs)--pluripotent stem cells derived from the inner cell mass of the blastocyst of the early embryo in vitro-the simultaneous expression of cell surface receptors and transcription factors complicates the detection of PGCs. To date, only a few genes that mark the onset of germ cell commitment in the epiblast--the outer layer of cells of the embryo--including tissue non-specific alkaline phosphatase (TNAP), Blimp1, Stella and Fragilis--have been used with some success to detect PGC formation in in vitro model systems. Here, we identified 11 genes (three of which are novel) that are specifically expressed in male and female fetal germ cells, both in vivo and in vitro, but are not expressed in ESCs. Expression of these genes allows us to distinguish committed germ cells from undifferentiated pluripotent cell populations, a prerequisite for the successful derivation of germ cells and gametes in vitro.