Germline cells are the only cells that develop into eggs or sperm, while somatic cells develop into the body. Progenitors of the germline, known as primordial germ cells (PGCs), differentiate into eggs or sperm after embryonic development. The expression of a select group of genes occurs in the PGCs of a number of different animal groups, indicating a possible conserved mechanism of germline gene activation. Indeed, the conserved transcription factor protein Ovo is required for expression of these genes, but it was previously unclear if Ovo was needed for normal germline development of the fruit fly (Drosophila) or whether the related mouse protein played a similar role in mouse embryos.