Although mammalian sex determination is normally specified genetically by an XX or XY chromosome complement, germ cells develop as sperm or oocytes in response to molecular cues provided by the gonadal somatic cells. In an ovary, germ cells enter meiosis during fetal life, thereby committing to oogenesis. In a testis, germ cells do not enter meiosis until after birth, at puberty. Recent findings indicate that, in mice, the sex-specific timing of entry into meiosis is governed by the balance between 2 secreted signalling molecules, retinoic acid (RA), which promotes entry into meiosis, and fibroblast growth factor 9 (FGF9), which counteracts RA. The combined action of these 2 molecular regulators provides a safety mechanism to guard against germ cell dysregulation that can lead to infertility or germ cell cancers.