The recently identified scaffold protein RanBPM belongs to the Ran-binding protein family. Like other scaffold proteins, RanBPM interacts with numerous proteins, but what is its function? Lino Tessarollo and co-workers now report that RanBPM is essential for mouse gametogenesis (see p. 2511). Using gene targeting, the researchers show that adult RanBPM−/− mice of both genders are sterile and have atrophied gonads. They report that in male RanBPM−/− mice the testes develop normally for one week postnatally but that there is then a marked decrease in spermatogonia proliferation. The first wave of spermatogenesis, they report, is characterised by spermatocyte apoptosis towards the end of prophase I. Moreover, experiments in chimeric mice indicate that RanBPM acts in a cell-autonomous way in male germ cells. Finally, they show that fertility in female RanBPM−/− mice is compromised because of germ cell depletion at the end of prophase I. Thus, mammalian RanBPM plays a crucial role in both spermatogenesis and oogenesis.