The primitive endoderm lineage of the mouse blastocyst: Sequential transcription factor activation and regulation of differentiation by Sox17

► Sequential activation of transcription factors within the PrE lineage. ► SOX7 is localized to sorted PrE cells starting from the salt-and-pepper stage. ► SOX7 localization is unaltered in Sox17 mutant blastocysts. ► SOX17 required for PrE integrity in implantation delayed blastocysts.