Baltimore, MD. The recent findings reported in Nature (March 11, 2004) by Jonathon Tilly’s group at Harvard Medical School, show that female mice produce stem cells that give rise to eggs. This result overturns previous notions about mammalian reproduction, which held that females are born with all the eggs that they will ever have and that the decline in egg quality that occurs after a certain age is due to an extended aging process. What mammalian research has not been able to address at this point, however, is how these stem cells operate, what prompts them to develop into eggs, and why they are eventually lost. To answer these questions we must turn to our cousin the fruitfly.