In mammalian embryos, the supporting cells in the developing gonads (which differentiate into the Sertoli and granulosa cells of the testes and ovaries,respectively) `tell' the germ cells which sex to become. Retinoic acid is thought to control this process by inducing meiosis in the developing ovaries but not in the testes where it is metabolised. However, on p. 1415, Best and colleagues results' support an alternative theory: that a meiosis inhibitor exists in mouse embryonic testes. The researchers identify Sdmg1, a conserved transmembrane protein that is expressed in mouse embryonic Sertoli cells at the time of germ-cell masculinization. Knock-down of Sdmg1 in Sertoli cell lines disrupts membrane trafficking and impairs their ability to masculinize germ cells, they report. Furthermore, perturbing secretion with brefeldin A in male embryonic gonads in organ culture causes male-to-female germ cell sex reversal. The researchers propose, therefore, that Sertoli cells communicate male-sex determining decisions to the germ cells during development by secreting either a meiosis inhibitor or other signalling molecules.