Germline cells transmit genetic information from generation to generation, making them the focus of reproductive studies. In mice, primordial germ cells (PGCs) appear in gastrulating embryos at embryonic day 7.25 and have been generated in vitro from the outer germinal membrane of an ovum (epiblast), provided the epiblast is cocultured with cells expressing bone morphogenic protein (BMP) 4 and 8b. How the founder population of PGCs is separated from the rest of the pluripotent epiblast has been unclear because of technical difficulties in distinguishing PGCs from embryonic stem (ES) cells, since standard PGC markers (OCT4 and alkaline phosphatase) are also positive in ES cells. In the September 15 PNAS, Yayoi Toyooka and colleagues at the Mitsubishi Kagaku Institute of Life Sciences use the mouse vasa homolog (Mvh) as a molecular marker for PGCs and show that ES cells can form these germ cells in vitro (PNAS, DOI:10.1073/pnas.1932826100, September 15, 2003).