Many genetic and cellular changes occur as pluripotent mammalian epiblast cells develop through primordial germ cells (PGCs) into functional gametes. However, little is known about when and how PGCs acquire the capacity to differentiate into functional germ cells. Now, Chuma and co-workers show that male germ cell development unexpectedly does not require exposure to the somatic cells of the foetal gonad (see p. 117). In transplantation experiments, epiblast cells harvested from 6.5-day-old mouse embryos establish spermatogenesis after being transferred into immature postnatal testes of infertile mice, and PGCs transplanted from 8.5-day-old embryos onwards develop into fertile spermatozoa. Furthermore, the erasure of parental methylation that occurs in mid-gestation PGCs occurs in the transplanted cells, indicating that this important developmental event is programmed autonomously in the PGCs. The researchers conclude that male germline differentiation is remarkably flexible and can occur in the postnatal testis.