Mammalian germ cell lineages begin with primordial germ cells (PGCs), which generate spermatozoa in males and oocytes in females. Last year, Mitinori Saitou and colleagues at the Institute for Integrated Cell-Material Science at Kyoto University in Japan showed that ESCs and iPSCs from male mic could be turned into primordial germ cell-like cells (PGCLCs) that become viable spermatozoa. Saitou wanted to see if they could repeat the trick to produce eggs, transforming stem cells into PGCLCs that could later become oocyte precursors, which are far less abundant in nature than precursors for spermatozoa.