How to make a human germ cell

How the primordial germ cell (PGC) lineage, which eventually gives rise to spermatozoa in males and oocytes in females, is established in the developing mammalian embryo has been a critical topic in both developmental and reproductive biology for many years. There have been significant breakthroughs over the past two decades in establishing both the source of PGCs and the factors that regulate the specification of this lineage in mice,1 but our understanding of the factors that control PGC development in the human is rudimentary. The SRY-related HMG-box (SOX) family of transcription factors consists of 20 genes in humans and mice that are involved in the maintenance of pluripotency, male sexual development, and other processes. A recent paper in Cell has identified one member of this family, SOX17, as an essential factor for inducing the PGC lineage in humans.2 Surprisingly, this protein does not appear to have a role in PGC specification in mice. This work not only introduces a new and important player to the field of germ cell specification, but also emphasizes the uniqueness of human PGC development compared to more extensively studied mouse models.